Uptake of mutagenic substances by persons handling injectable antineoplastic agents was studied, and various methods of preventing such exposure were evaluated. Six persons who prepared i.v. admixtures individually collected their urine in 24-hour batches during several eight-day study periods; each prepared from 12 to 90 admixtures per working day during each period. I.V. personnel prepared admixtures in both horizontal laminar-flow hoods and vertical-flow biological-safety cabinets, and using gloves and masks. Three pharmacy personnel who did not handle any drugs served as controls. Urine was concentrated (each 24-hour sample was concentrated to about 1 ml), and the Salmonella/mammalian microsome mutagenicity test (Ames test) was used to detect mutagenic activity in the urine concentrates. Mutagenicity was observed in the urine of all personnel during periods when they prepared antineoplastic admixtures in horizontal laminar-flow hoods, both when using no protection and while wearing gloves or masks. When admixtures were prepared in the vertical-flow biological safety cabinet by personnel wearing gloves, no urine mutagenicity was detected. The control group showed no urine mutagenicity. Protective intervention methods should be used by all persons handling antineoplastic agents. Guidelines for handling these drugs are presented.