Modifications of HLA antigens (transient loss or polyreactivity) were found in thirty-two of fifty-five examined patients with blood diseases (leukaemia, reticulosis, reticulo- and lymphosarcoma, lymphogranuloma, erythroblastoma) and malignant tumours. In two cases of acute myelosis transient alteration of HLA-A 2 antigen in HLA-A 28 was demonstrated. The relationship of HLA antigenic modification to chemotherapy was established, both by analysis of individual cases and by statistical evaluation of serological results in patients with or without chemotherapy (0-01 greater than P greater than 0-001). Chemotherapy is thus one of the causes for the development of HLA antigen modifications in blood diseases and malignant tumours.