Previous work with the autoradiographic mutant 21 former tobacco smokers had a mean variant lymphocyte assay has provided information about (mutant) frequency (Vf { standard error) of 1.97 the time-course of development of hprt mutations ({0.13) per million evaluatable cells. The Vf of and the persistence of detectable mutant cells in 42 subjects who had never smoked was 1.74 human subjects following therapeutic exposures to ({0.13) 1 10 06 , not significantly different from the genotoxic agents. These early studies also revealed former smokers. The smokers had Vfs of 8.09 elevations in frequencies of mutant cells in pretreat-({0.78) 1 10 06 for 18 heavy smokers and 5.22 ment blood samples from patients who were current ({1.02) 1 10 06 for five light smokers. The two catetobacco smokers, but no information was available gories of smokers had frequencies of mutant cells on former smokers. In the present study, blood sam-significantly different from each other, and each ples were obtained from 21 healthy former tobacco was significantly higher than non-smokers and forsmokers who had quit smoking at least 1 year be-mer smokers (P Γ΅ 0.05). Vfs were significantly corfore sampling, 42 subjects who had never smoked, related with both cotinine concentrations and the and 23 tobacco smokers. Plasma from all samples number of cigarettes smoked per day, P Γ΅ 0.001. was tested for cotinine, a metabolite of nicotine. This study demonstrates the sensitivity of the autora-Current smokers were categorized as heavy smok-diographic hprt assay for detecting mutagenic ers (Β’10 cigarettes per day, cotinine Β’90 ng/ml effects related to chronic low-level exposures to genplasma) and light smokers (Γ΅10/day, cotinine Γ΅ otoxins, and indicates that this assay is more likely 90 ng/ml). Lymphocytes from the blood samples to detect the effects of recent rather than past exwere isolated, cryopreserved, and later thawed and posures. Environ. Mol. Mutagen. 30:131-138, assayed with the autoradiographic hprt assay. The 1997.