Claims by Richard Harwood and Ditlieb Felderer
Harwood/Felderer’s first claim:
It would thus appear that the entire [soap] allegation is founded on anonymous reports and speculative hearsay. No one can come up with any locations, dates, or names. 
While many of the early reports regarding human soap were anonymous, usually regarding the RIF soap, the later ones regarding the Danzig Anatomical Institute are not based on anonymous reports. Although they mention the photograph of the IMT soap evidence reproduced in Butz’s book, Harwood and Felderer fail to mention USSR-197 (affidavits of Sygmund Mazur, 28 May 1945; 11 June 1945; 12 June 1945); USSR-264 (affidavit of John Henry Witton, 3 January 1946); or USSR-272 (affidavit of William Anderson Neely, 7 January 1946).
They also fail to mention anything regarding Professor Spanner, Mazur, Witton, and Neely. All actually worked in the Danzig Institute — not quite “speculative hearsay.”
Harwood/Felderer’s second claim:
Many Exterminationist books make no mention at all of the “soap” story; even outlandish books such as We Have Not Forgotten (2 & 6) which covers every other conceivable German atrocity. The same is true of the numerous other “Holocaustiana” which I have plowed through. Surely if there were such factories there would be ample evidence to write book after book, article after article, on this one subject. 
This is in direct contradiction to Mark Weber who argues (see above) that the soap story “has been authoritatively endorsed by numerous historians.” Yet his two fellow Holocaust-deniers are pointing out that most “exterminationists” do not mention the soap allegations in their books and use that fact as proof that it did not happen. That is hardly the picture that Weber painted in his article. The “revisionists” can’t have it both ways.
Harwood/Felderer’s third claim:
Determined to get to the bottom of the “human soap” problem, I paid a visit to Danzig, and unsuccessfully tried to locate the site of the “human soap factory.” At the nearby Stutthof “extermination camp” I again sought evidence, but not one of the officials or guides there could help. 
Evidently, Felderer and/or Harwood did not go to the Medical Academy on that visit to Gdansk. Other researchers have not had any such problems. Carl Tighe discussed Mazur, Spanner, and the Institute in his book, Gdansk: National Identity in the Polish-German Borderlands, and in a letter to the authors, he wrote:
I lived in the city of Gdansk in 1975-76 and was shown the recipe Spanner used – I believe it is still in the possession of the Polytechnic. Among students, particularly students at the Medical Academy and Polytechnic, and local residents, most of whom arrived in the city after Spanner had left, Spanner’s experiments were common lore. 
Julian Hendy visited the Gdansk Medical Academy during the Summer of 1994. According to Hendy, “It occupies the same building as the Anatomical Institute, the small brick shed built by the British POWs is still there. And there’s a plaque on the wall about the soap experiments.”
Harwood/Felderer’s fourth claim:
It is certain that if the western public realized that almost all of these atrocity allegations emanated from the communist bloc, then they would receive about as much credence as contemporary communist propaganda about intervening in Hungary, Czechoslovakia and now Afghanistan to “rescue the inhabitants from foreign interference.” 
Although it was a Russian (L. N. Smirnov) who brought up the soap allegations at the IMT, the Soviets had no control over the British statements. Both Neely and Witton gave their depositions to the British Judge Advocate General’s Office — in fact, both USSR-264 and USSR-272 clearly bear the designation MD/JAG/FS/22/609(4a) across the top.
What about Mazur’s depositions? Were they just communist propaganda, or can his statements to the Soviets (USSR-197) be corroborated by anyone else? Before speaking to the Soviets and giving his depositions, Mazur was interviewed by the Glowna Komisja Badania Zbrodni Niemieckich w Polsce (“Committee for the Investigation of German Crimes in Poland”).
This Committee, which was comprised of several prominent Poles (journalists, doctors, lawyers) as well as some representatives of the Red Army, entered the Danzig Institute on May 5, 1945. Mazur gave his formal deposition to the Committee on May 12, sixteen days before he gave his first deposition to the Soviets.
Zofia Nalkowska, a prominent novelist, was a member of the Committee and discussed Mazur, Spanner, and the Danzig Institute in her 1946 non-fiction book, Medaliony. The relevant portion was translated into English in Introduction to Modern Polish Literature, Ed. Adam Gillon and Ludwik Krzyzanowski. Nalkowksa quotes extensively from Mazur, and what he said to the Committee was in substance exactly what he later said to the Soviets. Nalkowska in no way can be considered a communist tool.
Stanislaw Strabski, another member of the Committee, was a Polish journalist and published a 1946 book called Mydlo z ludzkiego tluszczu, a preliminary translaton of which shows that he also discusses Spanner, Mazur, and the Institute. So it is disingenuous to merely dismiss the testimony at the IMT regarding the soap as communist propaganda: two of the three affidavits were provided by the British JAG, and Mazur’s statements to the Soviets are consistent with what he told the Committee earlier in May 1945.