The human monoclonal antibody Ha6D3 of the IgM type was used to stain malignant lymphoma cells from peripheral blood in flow cytometry and from cryosections of lymph nodes using the immunoperoxidase technique. It was found to react with peripheral white blood cells of all 12 cases of leukaemia and with lymph node cells of seven out of 11 B cell lymphomas and with the one T cell lymphoma tested so far. For in vivo experiments a batch of 70 mg Ha6D3 was purified and 6 mg Ha6D3 was injected intravenously into a chimpanzee with time intervals of 10 months and 1 month. The side effects observed were shivering, some muscular spasms and variations in the heart frequency. A decrease of lymphocytes of more than 50% was documented by haematogram analysis. The flow cytometry data showed that the Ha6D3 antigen does not modulate. Even after three repeated injections applied in a time interval of several months no immune response to Ha6D3 could be detected in vivo or in vitro. Based on these data we suggest that Ha6D3 may become a candidate for the treatment of certain leukaemias in vivo.