A series of studies were undertaken to evaluate the chemotherapeutic response to various antineoplastic drugs of human breast (MX-1) or colon (CX-2) tumor xenografts growing in genetically athymic (nude) mice. Fragments (2mm3) of either tumor type were implanted subcutaneously into the subaxillary region of NIH Swiss nude mice, and single drug therapy was started when tumors became palpable and were growing progressively. 5-Fluorouracil (5-FU) administered on a Q7DX3 schedule starting on Day 21 post tumor implantation elicited significant retardation in the growth rate of CX-2 tumor. A single treatment with methyl-CCNU induced temporary tumor regression. Against MX-1 tumor, both cyclophosphamide and melphalan induced tumor regressions with no recurrence. 5-FU slowed but did not arrest growth of MX-1 tumor. These tumor systems grown in nude mice appear to be suitable models for in vivo screening of anticancer agents that would prove clinically active.