The haematologic effects of radioimmunotherapy (RAIT) in cancer patients have been studied in order to better understand the aetiology of RAIT-associated myelosuppression. Evaluations were performed on patients treated with 131I-anti-carcinoembryonic antigen (CEA) and 131I-LL2, a monoclonal antibody (MAb) reactive with non-Hodgkin's B-cell lymphoma. Both groups of patients experienced decreases in WBC and platelets. Nadirs were observed 42-51 d post first injection in the lymphoma patients, and 49-66 d post first injection (30-43 d post high-dose therapeutic injection) in the carcinoma patients. Within the WBC population, B cells were the most radiosensitive. The evaluations of the binding of these MAbs to peripheral blood cells and the effect of RAIT on lymphocyte subpopulations indicated a drop of 26-92% in the percentage of B lymphocytes within 1 week following treatment with both 131I-LL2 and 131I-anti-CEA MAbs even though only LL2 binds to B cells. The percentage of T lymphocytes was not affected by the 131I-antibody treatments. These observations suggest that the marked drop in circulating B lymphocytes and platelets after the administration of radioiodinated antibodies is a nonspecific radiation effect, and not necessarily related to the binding of MAb to normal B cells. Thus, among WBCs, B cells are uniquely radiosensitive, and will be unusually affected by antibody-directed internal radiation.