The first two transports of Hungarian Jews bound for Auschwitz set out from the Kistarcsa camp near Budapest and the town of Topolya in Vojvodina on 28 and 29 April, respectively. They arrived at the so-called old Jewish ramp on 2 May: of the 3,800 persons only 486 men and 616 women were directed as prisoners to the camp, the rest were murdered in the gas chambers. It was then that it became apparent that Auschwitz was not ready to receive such large numbers of deportees: a railway siding together with a ramp on which selections could be conducted within the camp had been under construction for months but were not yet completed, while of the two bunkers/makeshift gas chambers only one still functioned and of the four crematoria one (No. 4) was now for some time out of use due to technical failures. Responsibility for this state of affairs was ascribed to the commandant of the SS garrison, Arthur Liebenhenschel. He was therefore replaced by Rudolf Höss, especially brought over from WVHA headquarters in Oranienburg on 9 May. At the same time the commandant of Birkenau, Friedrich Hartjenstein, was replaced by Josef Kramer, who arrived from Natzweiler concentration camp.

After taking up his duties, Höss vigorously proceeded to complete construction of the Birkenau ramp and reactivate the gas chambers in bunker II (‘the little white house’). Near it he had pits dug for the burning of corpses; similar pits were also dug next to the gas chambers of crematorium V. Soon afterwards he transferred from a sub-camp in Gliwice Otto Moll, who had supervised the burning of corpses in pits next to Birkenau bunkers I and II in 1942. Finally, towards the end of May he ordered the setting up of three wooden barracks next to bunker II to serve as undressing rooms for Jews selected to die. At the same time the Birkenau Sonderkommando was increased from 420 to 620 prisoners.