Six drugs, three of which are considered to be active against human breast cancer [melphalan (PAM), cyclophosphamide (CTX), and 5-fluorouracil (FUra)] and three of which have failed to demonstrate activity against human breast cancer [N-phosphonacetyl-L-aspartate (PALA), cytarabine (ara-C), and 6-thioguanine (TG)], were tested at optimal weekly doses in (BALB/-cfC3H × DBA/8)F1 (CD8F1) mice bearing spontaneous, autochthonous breast tumors averaging 300 mg. When treatment was evaluated by laboratory criteria (i.e., tumor growth inhibition in comparison to vehicle-treated, size-matched controls), all six of the drugs tested were judged to be active. However, when the criteria for positive drug activity consisted of the attainment of tumor regressions of ≧50% in ≧20% of the treated individuals (i.e., analogous to clinical criteria), only the three drugs that are known to be active against human breast cancer (PAM, CTX, and FUra) were judged active against the spontaneous murine breast tumors. PALA, ara-C, and TG failed to demonstrate regressing activity against the spontaneous murine breast tumors. With a cavea concerning the limited spectrum of drugs evaluated in this study, it can be concluded that the CD8F1 breast tumor model demonstrated 100% correlation with human breast cancer in terms of both positive and negative drug sensitivity when the criteria for evaluation were parallel. [J Natl Cancer Inst 1988;80:52–55]