Ten healthy male and 10 healthy female 'never-smoking' subjects (ages 21-50) participated in a 5-day environmental room study to determine if an acute exposure to a high level of fresh diluted sidestream smoke (FDSS) would alter urinary mutagenicity. On Monday, Tuesday, Thursday and Friday, the 20 subjects sat in environmental rooms for 7.33h and were exposed to filtered and humidified air. On Wednesday, the 20 subjects were exposed in the environmental rooms for 7.33h to an average respirable suspended particle (RSP) concentration of 179 microg/m(3) of FDSS generated by machine smoking 1R4F Kentucky reference cigarettes. This level of FDSS is approximately three times the ETS level seen in the top 5% of US workplaces which allow smoking. A cumulative 7.33h air sample from each environmental room was collected and determined to be mutagenic by Ames Salmonella assay. Subjects' urinary mutagenicity was measured on Wednesday as compared with Tuesday or Thursday by assaying concentrates of 24h urine samples in Ames Salmonella bacterial strains TA98 and YG1024. Diet was strictly controlled on all study days, with broiled and pan-fried meat not served to minimize ingestion of mutagenic protein pyrolysis products. Although all the urinary mutagenicity values were within the range reported for minor changes in diet, the subjects experienced a small but statistically significant increase (p<0.05) in urinary mutagenicity in strain YG1024, but not in the less sensitive strain TA98 on the day of FDSS exposure.