Since 1972, the National Surgical Adjuvant Breast and Bowel Project (NSABP) has carried out a series of clinical trials evaluating the worth of adjuvant chemotherapy in the management of patients with primary breast cancer. This report provides information concerning (a) protocol compliance relative to drug administration and (b) acute toxicity encountered by patients in three separate trials who were given one-, two- or three-drug chemotherapy within 1 month of operation. The findings are derived from 1548 women who received 20,765 courses of chemotherapy, the most extensively documented experience yet reported. They indicate that despite the large number of physicians and the heterogeneity of the institutions participating, large cooperative efforts can be accomplished with credibility. Only 13 (0.8%) of the women failed to complete all courses of therapy for reasons directly related to nonprotocol compliance by physicians. Only 4.3% failed to complete therapy for miscellaneous reasons other than toxicity, treatment failure, occurrence of a second primary, or death unrelated to tumor. While almost all patients experienced toxic reactions during the therapy, only 3%--4% of recipients of melphalan (L-PAM; P) and 4%--5% of recipients of L-PAM + 5-FU(F)(PF) failed to complete 2 years of therapy because of toxicity. Of those patients receiving PF + methotrexate (MTX; M) (PMF), 15% did not finish their treatment for that reason. While there was little difference in hematologic and nonhematologic toxicity between those patients receiving P or PF, and such toxicity was generally acceptable to both patients and physicians, the addition of MTX (PMF) resulted in greater toxicity (vomiting, stomatitis, and alopecia) which was less readily accepted. Tolerance of any of these regimens was unrelated to patient age, despite the belief that older women are less tolerant of chemotherapy. The earlier toxicity occurred, the greater was the number of subsequent courses associated with toxicity, and the lower was the total amount of drug received. The extent of the toxicity produced by the NSABP regimens and the end results obtained with them, must be compared with the end results and toxicity obtained by other regimens before making a choice of the adjuvant therapy to be used.