The development of a criteria-based antimicrobial formulary system for use in a 432-bed university-based tertiary-care hospital is described. A subcommittee of the hospital's pharmacy and therapeutics committee developed prescribing criteria for antimicrobials thought to have a specific place in the treatment of various infections. The criteria were based primarily on patient needs, but when two therapies were thought to have similar effectiveness and similar toxicity profiles, cost became the deciding factor in drug selection. Educational packets were designed to assist physicians in prescribing criteria-based antimicrobials. An antimicrobial order form was also developed. The program was implemented after an extensive orientation and education period; data on antimicrobial use were collected for the first year of the program. Hospitalwide compliance with the criteria was 89%, although compliance in certain departments was poorer. The database was formulated to be both service- and physician-specific for the purposes of reporting and education.