Treosulfan (L-threitol-1,4-bismethanesulfonate, Ovastat) is a bifunctional alkylating agent that shows a formal structural similarity to busulfan and is applied clinically to patients suffering from ovarian cancer. The present study demonstrated the pronounced antitumor activity of this drug against three of five human breast carcinomas xenografted to athymic mice. It was shown that treosulfan is capable of inducing irreversible and complete remission of the heterotransplanted human breast carcinomas MDA-MB-436 and MX-1 within 14 days after drug application and of effecting growth inhibition by more than 90% in the MDA-MB-435S xenograft. In all three carcinomas, treosulfan caused more pronounced growth reduction than did equitoxic doses of the alkylator cyclophosphamide. Adriamycin, an intercalating cytostatic agent that is an important component of clinical nonhormonal chemotherapy of breast carcinomas, induced only partial remission of these three xenografts and inhibited the tumor growth by 80%-90% (MDA-MB-436, MX-1) and by 70%-80% (MDA-MB-435S), respectively. In the M 3 xenograft, treosulfan just led to a retardation and stagnation of tumor growth; it was again more effective than Adriamycin but was clearly less active than cyclophosphamide. The FM 2 breast carcinoma, finally, was the only xenograft whose growth was not influenced by treosulfan at doses up to that which was lethal to 50% of the treated mice (LD50 value). These results confirm that treosulfan is effective against human breast carcinomas. Because of this activity as well as the known low toxicity and good clinical compatibility of treosulfan, it should be considered for introduction into nonendocrine chemotherapeutic regimens against human breast carcinomas and investigation in clinical trials.