Ninety patients with advanced breast cancer received a polychemotherapeutic program composed of 5-fluorouracil, cyclophosphamide, and prednisone with or without vincristine as a control group in a series of four consecutive Phase II clinical trials of new drug programs. Objective regression rates were 59% without vincristine and 46% with vincristine. Projected mean length of regressions exceeds I year. Site of dominant disease, disease-free interval, or performance scale score (if score was 0, 1, or 2, ECOG scale) failed to influence response rates; decreasing response rates were noted as the length of time increased after menopause. No advantage existed in patients experiencing severe myelosuppression (nadir leukocyte count of less than 1,500/mn3, and appreciable response rates occurred without significant myelosuppression. The addition of vincristine to the regimen failed to increase the response rates, and only increased toxicity. The program as outlined is reasonably tolerable and effective for this group of patients with advanced breast cancer.