AbstractDirect in vivo tests of somatic cell mutation in man may provide realism in assessing the genetic risks of potential environmental mutagens. The autoradiographic determination of purine analogue (8‐azaguanine; 6‐thioguanine) resistant (AGr; TGr) peripheral blood lymphocytes (PBLs) arising in vivo in man is proposed as a candidate test. PBLs bearing the naturally occurring Lesch‐Nyhan (LN) mutation are prototype mutant cells. LN PBLs are AGrand TGr, whereas normal PBLs are AG and TG sensitive.When judged by the inhibition of phytohemagglutinin (PHA) stimulated3H‐thymidine incorporation in vitro, analogue‐resistant LN PBLs may be distinguished from analogue‐sensitive normal PBLs by several methods. Early studies quantitating PHA stimulation by scintillation spectrometry detected down to 1% of LN PBLs in artificial mixtures with normal PBLs. Although LN heterozygous females could be identified on the basis of lymphocyte mosaicism, scintillation spectrometry was too insensitive to detect rare “LN‐like” PBLs in non‐LN individuals. Autoradiography, however, detected rare TGrPBLs in normal non‐LN individuals. Their frequencies did not increase with age. With this method, TGrPBL frequencies in LN heterozygous females were found to range from 1 × 10−3to 5 × 10−2, whereas blood samples from LN males showed from 23% to 100% TGrcells. Rare LN PBLs could be detected in artificial mixtures with normal cells.Studies in human patients undergoing various potential mutagenic therapies assessed the effects of these therapies on the TGrPBL variant frequencies (Vf) of non‐LN individuals. Group TGrPBL Vfvalues were higher in treated patient groups than in controls. However, some untreated patient groups (cancer and psoriasis) also had elevated values, suggesting that disease itself may affect TGrPBL frequencies. Nonetheless, one patient group (vitiligo) showed elevated Vfvalues in treated (8‐methoxypsoralen and long‐range UV light = PUVA) but not in untreated patients, suggesting that treatment was responsible for the TGrPBL elevations. Longitudinal studies over time in cancer patients receiving X‐irradiation therapy demonstrated that such exposures also are associated with TGrPBL frequency rises and suggested that longitudinal studies may be necessary to relate TGrPBL Vfelevations to specific environmental influences.Variant TGrPBLs were found at frequencies comparable to those in man in the peripheral blood of rats. They increased in a single study following treatment of the animals with a clinical alkylating agent.Characterization of the TGrPBLs suggests that some of these cells are mutants. Presumably the mutant cells arise in vivo by somatic cell mutation.There also appears to be a population of nonmutant cells that incorporate3H‐thymidine under conditions of the current assay (phenocopies). Current efforts are directed at measures that will allow phenocopies to be distinguished from mutant PBLs.The autoradiographic method is presented as one potentially capable of direct mutagenicity testing in man. It is suggested that its value for this, and the relevance of somatic cell mutation to human health, be tested by correlating sequential mutagenicity test results with eventual clinical outcomes in patients recei