Leukaemic cells from four out of 19 patients with acute myeloblastic leukaemia are shown to be resistant to lysis by rat anti-muman-myeloblast antibody in the presence of complement. To a lesser degree this is reflected in lack of immunogenicity of these myeloblasts. It is suggested that these findings may be relevant in planning immunotherapy in this disease and evaluating the benefit of this type of treatment. Finally data are presented to show that loss of immunogenicity of myeloblasts after irradiation with 10,000 R, as is practised in certain immunotherapy schedules, is small compared with differences in immunogenicity between non-irradiated myeloblasts from different individuals.