Sera from 42 patients with acute myelogenous leukaemia, who were receiving immunotherapy of weekly injections of allogeneie blast cells and bacillus Calmette-Guérin, have been tested for cytotoxic antibodies. In 5 of the 16 patients with antibodies, it was possible to show that these detected HL-A specificities present on the immunising cell. Eight out of 15 patients immunised for more than 12 months produced antibodies against a panel of unrelated normal lymphocytes. The survival time did not differ significantly between patients who produced antibody and those who did not.