The Inter-Society Committee on Antimicrobial Drug Usage has conducted a survey of 20 randomly selected general hospitals in Pennsylvania. Records studied were of 5288 patients who either died or were discharged on 10 random days throughout 1974. About 28% of these patients received at least one antimicrobial drug during hospitalization. In 60% of the patients the drugs were given for treatment of infectious disease; in 30% they were given as prophylaxis for surgical or nonsurgical procedures. Prophylactic treatment was given for more than 2 days after a procedure in most cases, even though it is not considered effective after 48 h. Discontinuing prophylaxis after 48 h would reduce by 20% the use of antimicrobial drugs in hospitals. Overall, ampicillin and the cephalosporins were the most commonly prescribed drugs, with the more toxic drugs being used infrequently; over half of the patients given antimicrobial drugs had cultures taken. Both of these findings are consistent with accepted standards for antimicrobial drug usage.