Serum and tissue concentrations of bestrabucil (KM 2210), a combined agent of 17-estradiol and Chlorambucil, were examined in patients with urogenital cancers including those of the kidney, bladder, prostate and testis. We administered orally 100mg (50 mg X 2/day of bestrabucil for 3 days), and determined its plasma levels and metabolites. A maximum drug concentration, i.e., 9.25 ng/ml, was noted 3 hours after administration; a constant plasma level of 5 ng/ml was maintained and the concentration of free chlorambucil was low. After single or consecutive oral administration of bestrabucil (100-300 mg), tumor specimens contained significantly large amounts of bestrabucil in comparison with adjacent normal tissue. Selective accumulation of the active component in tumor tissue suggests the clinical usefulness of bestrabucil.