Class I and II HLA typing was investigated before and at various intervals after a 10 Gy total body irradiation delivered over 4 h, prior to allogeneic bone marrow graft for various hematological malignancies, in 14 patients. A reliable class I HLA typing appeared to be possible in almost all cases 6-8 hours after the start of irradiation but was only possible in 5 patients after 24 h. Preliminary results with class II antigens might suggest a more marked "fragility" of this antigen class after irradiation. These results encourage the drawing of blood samples for HLA grouping as soon as possible after accidental whole-body irradiation.