The hematologic factors of 16 female patients with breast carcinoma who had received adjuvant chemotherapy with L-phenylalanine-based programs for two years were compared with those for 13 normal female volunteers of similar age. The findings include significant suppression of the mean hemoglobin level, of leukocyte counts, and of circulating neutrophils and lymphocytes in patients 3-27 months after chemotherapy. Marrow neutrophil reserve studies with prednisolone stimulation again demonstrated significant suppression for the patients who had received chemotherapy. The authors conclude that there is a chronic hematologic toxicity associated with this therapy.