In a cohort of 764 evaluable patients with primary breast cancer, we have compared the ability to deliver full doses of adjuvant chemotherapy in two patient groups: one undergoing conservative breast surgery plus irradiation and the other having modified radical mastectomy as primary treatment for the cancer. We have also analyzed the toxicities of the concurrent radiation and chemotherapy. The group having irradiation had significantly more moderate leucopenia, which caused a short delay (median, three weeks) in the overall time necessary to complete the planned chemotherapy. However, among those patients who completed the planned chemotherapy cycles, the fraction who received more than 85 percent average drug doses was 96 percent or higher in all but one small subgroup. Interaction between the irradiation and chemotherapy caused mild breast skin reactions in 42 percent of patients so analyzed and worse reactions in 12 percent. When follow-up tracings were performed, mild electrocardiogram abnormalities occurred in 19 percent of patients, apparently because of the irradiation. We conclude that intravenous adjuvant chemotherapy, as administered in this study, can be delivered as intensely with conservative primary treatment as after mastectomy and that toxicity is mild, rarely requiring intervention or treatment discontinuation.