Mutagenicity in the urine of workers occupationally exposed to mineral oils and iron oxide particles and age matched workers only exposed to mineral oils was investigated using the Salmonella/mammalian microsome assay. Both groups of workers included smokers and non-smokers. Mutagenicity was significantly higher in the group of workers exposed to both mineral oils and iron oxide particles, the statistical significance of the difference being similar to that found when total non-smokers were compared with total smokers irrespective of occupational exposure. When only non-smokers of both groups of workers were compared, the extent of mutagenicity in the urine of workers exposed to iron oxide particles was still significantly higher, suggesting that smoking did not exhibit a significant enhancing effect on urinary mutagenicity of workers exposed to mineral oils and iron oxide particles, but instead seemed to enhance urinary mutagenicity similarly in both groups of workers. Whether or not this conclusion can be drawn may depend, among other factors, on the variability of the Ames assay. To cope with this particular problem, the possible usefulness of a mutagenicity factor is discussed.