Twenty-one patients were entered for a clinical study of KM 2210, the benzoate of an estradiol-chlorambucil conjugate, in the treatment of hematopoietic malignancies. These included 4 cases of chronic lymphocytic leukemia (CLL), 5 cases of chronic myelogenous leukemia (CML), 5 cases of malignant lymphoma (ML) and 7 cases of multiple myeloma (MM). Twelve cases had prior chemotherapy. KM 2210 was given orally at a dose of 50-300 mg daily. Of 19 evaluable cases, two cases with CLL achieved complete response and 6 cases including 2 with CLL, 2 with CML and 3 with ML achieved partial response. There were no responders among the cases of MM. The partial and complete response rate was 47%. Toxicity included mild breast or nipple pain (28.6%), genital bleeding (9.5%), appetite loss (9.5%) and gynecomastia (4.8%). These side effects may have been due to increased levels of estrogen. Hematopoietic toxicity was mild and well tolerated. No cardiac, hepatic or renal toxicity was observed in this study. These results suggest that KM 2210 might be an effective candidate for the treatment of hematopoietic malignancies, especially CLL.