A rational approach to the proper use of antibiotics must include knowledge of microbial sensitivities to various agents, proper alternative choices, pharmacologic considerations, and the cost to patient and hospital of drugs prescribed. A logical program for analysis of use of antibiotics and proper administration is presented. Since the largest misuse of antibiotics is for no infection or for prophylaxis before surgery, situations in which prophylactic use of antibiotics is of value have been reviewed. Such an educational program designed to improve the physician's use of antimicrobial agents must be a continuing effort in improving patient care.