Mutagenic activity of urines obtained from operating room personnel was assayed in the Ames Salmonella typhimurium/mammalian [rat] microsome system using 3 strains of histidine-dependent S. typhimurium, TA 1535, TA 1538 and TA 100. Two procedures were employed. In the 1st, 100 and 200 .mu.l aliquots of urine obtained from 28 subjects working in scavenged or unscavenged operating rooms were tested. In the 2nd, urine samples obtained from 13 physicians before and after starting an anesthesia residency and 250-fold concentrates of these samples were assayed. There was no statistically significant difference in urinary mutagenic activities between individuals working in scavenged and those working in unscavenged operating rooms. Urines of anesthesiologists collected before and after beginning training had similar mutagenic activities. Only heavy smokers had mutagenic urine. The majority of operating room workers do not excrete mutagens in the urine.