Human somatic cell mutations arising in vivo in peripheral blood lymphocytes (PBL's) are quantitatively detected by a test which enumerates variant purine analogue (8 azaguanine equals 8 AG; 6 thioguanine equals 6 TG resistant (AGr; TGr) Lesch-Nyhan (LN) like PBL's in non-LN individuals. AGr and TGr PBL's are recognized in vitro by their resistance to analogue inhibition of phytohemagglutinin (PHA) induced 3HTdr incorporation. Variants are enumerated autoradiographically. Recent studies have suggested that some apparently TGr PBL's detected autoradiographically are phenocopies, i.e., non-mutants. Increasing intervals in vivo in selective agent, changing the viability marker to 3HUdr (RNA synthesis) and insuring that PBL's are at the GO cell cycle phase at initiation of in vitro assay are all being employed to eliminate phenocopies. Human HLA loss variant PBL's arising in vivo will be studied in an analogous fashion. Quantitative measures for their enumeration will allow this marker system also to be used for direct human mutagenesis studies. HLA loss variant PBL's, and continuously growing T-cells established from them, may be used as reagents in HLA testing. Human T-PBL's are propogated in vitro using T-cell growth factor (TCGF) in both suspension and clonal soft agar cultures. TGr and HLA loss variant PBL's arising in vivo will be propogated in vitro with TCGF, the stability of the variant phenotypes will be assessed and appropriate characterizations to document the mutant nature of the variants will be performed. Continuously growing T-PBL's maintained with TCGF will provide targets for an in vitro mutagenesis system to parallel the in vivo systems. Rat PBL systems analogous to those described for humans are being developed.