Assay procedures have been developed for the determination of hypoxanthine guanine phosphoribosyltransferase (HGPRT) and adenine phosphoribosyltransferase (APRT) activity in erythrocytes and for HGPRT and inosinic acid dehydrogenase activity in fibroblasts. In normal erythrocytes the mean activity for HGPRT using hypoxanthine as substrate was 39.09 mμmols/hr/μl of cells. The mean value for patients with the Lesch-Nyhan syndrome was 0.003 mμmols/hr/μl. Nearly all of the obligate heterozygotes studied had normal levels of HGPRT activity in both erythrocytes and fibroblasts. The normal erythrocyte mean level of APRT activity was 23.56 mμmols/hr/μl. All subjects with complete or partial deficiency of HGPRT and heterozygotes had elevated APRT activity. There were no differences among the inosinic acid dehydrogenase (IMPDH) activities of fibroblasts from subjects who were normal, HGPRT-deficient, or hyperuricemic without HGPRT deficiency.