The study examined the effect of exposure to autopsy workers to formaldehyde using urinary mutagenicity testing with Salmonella typhimurium. A matched control group was also studied. Additional studies including the recovery of histidine from urine samples, the determination of histidine in urine samples and the effect of exogenous and urinary histidine on the mutagenicity assay were also carried out. Results showed that there was no increase in mutagenicity in the autopsy workers as compared to the control group. A large proportion of the exposed individuals had toxic urine while only two of the control group has similar toxic urine. However, the toxic material in the urine samples has been isolated and does not appear to be related to the formaldehyde exposure. In conclusion, exogenous histidine and certain growth factors present in human urine can affect the outcome of both plate incorporation and fructuation assays for mutagenicity and care must be taken when interpreting results of this type of mutagenicity studies.