Class I and II HLA typing was investigated before and at various inter
vals 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 o
nly 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-bo
dy irradiation.