The role of prolactin (PRL) in the physiological regulation of the immune system and in hematopoiesis is well known. There is also evidence of the significance of PRL in several pathological conditions such as autoimmune diseases and some malignancies, e.g. colon and breast carcinomas and also B cell malignancies. Multiple myeloma is known as a B cell malignancy. It is the result of malignant transformation of a single clone of neoplastic plasma cells that synthesize abnormal amounts of monoclonal immunoglobulins or immunoglobulin fragments. In our present studies, the possible expression of PRL in bone marrow cells obtained from diagnosed multiple myeloma (17 cases) or nonmyeloma (5 cases) patients was examined by the method of immunocytochemistry. Samples obtained from those multiple myeloma patients (13 cases) who had not received chemotherapy for 6 months prior to these studies showed a positive immunocytochemical reaction for PRL. Bone marrow smears of patients diagnosed with multiple myeloma who had received chemotherapy within 6 months of the study and also the smears of patients without diagnosed multiple myeloma failed to show a positive immune reaction for PRL. In the case of a patient who was examined prior to and also after a period of 3 months of chemotherapy, the PRL-immunopositive bone marrow cells had disappeared due to the treatment. According to the light microscopic analysis of the cell morphology, PRL-immunopositive cells in the bone marrow were mainly, but not exclusively, plasma cells. There was no correlation between the positive PRL staining of cells and the type of monoclonal immunoglobulin or the ratio of plasma cells detected in the bone marrow. Taken together, our results indicate a possible role of PRL in multiple myeloma. Further experiments are necessary to identify the prognostic value of PRL in multiple myeloma.