N-Acetyltransferase activities were determined in tumor (12 malignant and 6 benign) and control (non-cancerous) breast tissues from 18 female patients. The activities of matched 12 malignant tumor and control tissue cytosols showed 6 rapid, 4 intermediate and 2 slow acetylators based on p-aminobenzoic acid (NAT1) and sulfamethazine (NAT2) as substrates, Compared to the activities of slow acetylators, the rapid acetylators exhibited mean apparent V-max values about 5- and 50-fold greater for p-aminobenzoic acid and sulfamethazine, respectively. No correlation was observed between the blood and breast tissue N-acetyltransferase (NAT1 and NAT2) activities. When the mean apparent N-acetyltransferase activities of the malignant and benign breast tumor tissues were compared, the results showed an increased activity for both p-aminobenzoic acid (PABA) and sulfamethazine (SMZ) acetylation in the malignant tissues compared to benign ones, and also control tissues showed lower activities compared to tumor tissues. Moreover, the mean NAT2 activity was about 2-fold greater in the malignant tissues when compared to NAT1 activity.