The appropriateness of antimicrobial prescribing habits by resident physicians in a family practice center was evaluated. From a six-month period, 225 antibiotic prescriptions were reviewed retrospectively, in a three-phase study. In the first phase, two physicians determined the validity of the diagnosis and treatment for each case, based on criteria suggested by current literature. In the second phase, the pharmacist investigators compared the prescribed regimens with the established criteria for appropviateness of drug choice, daily dose, dosage interval, and duration of therapy. In the third phase, charts were reviewed to determine if microbial cultures had been ordered. The diagnosis was accepted in 89% of the cases; of those, drug therapy was indicated for 84%, an appropriate drug was prescribed in 89%, daily dose was appropriate in 72%, dosage interval was acceptable in 75%, and duration of therapy was appropriate in 59%. Microbial cultures were commonly ordered for pharyngitis, cystitis, pyelonephritis, and gonococcal urethritis. Cultures were not ordered for tonsillitis, nongonococcal urethritis, prostatis, and pelvic inflammatory disease. The prescribing patterns of a group of family practice residents were found to be in less than full compliance with standards in the literature. However, the importance of this finding is difficult to judge because there have been few such studies in ambulatory care settings and the validity of some of the criteria for appropriateness is not known.