The mutagenicity of captan and of streptozotocin was tested in vivo by reversion of hisG46 base-pair substitution histidine auxotrophs of Salmonella typhimurium in the peritoneal cavity or in blood, plasma or urine of rats or mice. Genetic response was determined by the frequency of revertants (quantitative test) or by the number of revertants per plate (semiquantitative test). In quantitative HMA captan gave negative results following 3 hourly 500 mg/kg s.c. doses or 1000 mg/kg oral dose in mice with the hisG46 mutant or 2000 mg/kg oral dose in rats with the hisG46, uvrB (TA1950) mutant. The positive control SZN induced many reversions at 0.5 mg/kg i.p. or 10 or 100 mg/kg oral doses. In semiquantitative in vivo blood or urine assays captan gave negative results after a 250 mg/kg oral dose with hisG46. SZN in the same experiment gave positive results in both semiquantitative and quantitative in vivo blood assays following 1000 mg/kg i.p. or 2000 mg/kg oral doses in the rat with TA1950. Rat blood mixed with captan for 45 min before adding TA1950 cells inactivated 1000 mug captan/ml but not 5000 mg/ml in the semiquantitative test. Corresponding figures in the quantitative test were 500 mu/ml and 1000 mug/ml. Rat plasma inactivated the mutagenicity of about 10 times less captan than rat blood. Human blood inactivated about as much captan as rat blood. The mutagenicity of captan was inactivated more efficiently than of SZN by blood. The results of the experiments suggested that captan's mutagenicity is probably inactivated by glutathione of the erythrocytes. Rat S-9 liver microsomal fraction also strongly decreased captan's mutagenicity in a semiquantitative test with the R factor, uvrB, hisG46 (TA100) mutant.