Rare thioguanine-resistant T lymphocytes, present in vivo in human peripheral blood, were isolated and grown in vitro as thioguanine-resistant cultured T cells. The conditions for their selection in vitro were such that thioguanine resistance had to have arisen in vivo. The mutant cells bore T-cell surface markers, maintained their thioguanine resistance in vitro in the presence or absence of selection, and were deficient in hypoxanthine-guanine phosphoribosyltransferase activity.