1. The present study was designed to investigate the effect of melatonin on the proliferation of normal lymphocytes and certain T-lymphomas and myelomas under in vitro conditions. 2. The results revealed that administration of 200 microM melatonin inhibited significantly the incorporation of [3H]thymidine into both normal mouse and human lymphocytes and T-lymphoblastoid cell lines. 3. On the contrary, melatonin provoked an increase of myeloma cell proliferation. 4. The influence of melatonin on hybridoma cell lines was negligible. 5. Collectively, these data demonstrated that the chief pineal indole affect selectively the processes of lymphoblastoid cell growth.