Bestrabucil, the benzoate of an estradiol-chlorambucil conjugate, was initially developed as a target-oriented anticancer agent for breast cancers with positive estrogen receptors, by conjugating a tumoricidal agent, chlorambucil, to a vehicle, estradiol. Further studies, however, revealed that regardless of the presence of estrogen receptors, bestrabucil selectively accumulates in malignant tumor cells. The unique feature of bestrabucil, selective affinity to tumor cells, was demonstrated in in vitro and in vivo studies. Significantly larger amounts of 3H-bestrabucil accumulated in malignant cells (3T3-SV40 transformed) than in normal cells (3T3). In vitro inhibition effects of bestrabucil on cell growth was observed only in malignant cells. Selective accumulation of bestrabucil in malignant tumor tissues was also demonstrated in in vivo experiments. After a single oral administration of 100 mg/kg of bestrabucil to female Wistar rats bearing Walker 256 carcinoma, bestrabucil accumulated significantly occurred in tumor tissues with little or no accumulation in normal tissues and blood. Anti-tumor effects and toxicities of bestrabucil and chlorambucil, were compared using Walker 256 carcinoma. Bestrabucil exerted its antitumor effects with little change in leucocyte counts in the peripheral blood, whereas chlorambucil showed significant side effects. Finally, selective accumulation of bestrabucil in malignant tumor tissues, was demonstrated clinically. Tumor specimens obtained during the operation of patients with various types of cancer, 24 hours after oral administration of 100 mg of bestrabucil, contained significantly larger amounts of bestrabucil compared with adjacent normal tissues. Clinical trials of bestrabucil are being carried out at present. Bestrabucil seems to be a promising target-oriented anticancer agent and deserves further investigation.