The inheritance of phosphoribosyltransferase (PRT) deficiency is X-linked recessive. By a radioautographic procedure showing the incorporation of hypoxanthine-3H or guanine-3H into nucleic acids, a process requiring PRT, cultured fibroblasts from carrier females exhibit mosaicism. This method is useful for the detection of the heterozygote in this disease. Amniotic fluid cells obtained by amniocentesis are of fetal origin and can be grown in tissue culture. Such cultures from the gestational ages of 9 to 33 weeks have PRT specific activity of 66.86 ± 8.08 (average ± SD) which is similar to the PRT specific activity (81.0 ± 24.9) in normal cultured fibroblasts. Cultured amniotic fluid cells have been used for antenatal diagnosis of this disorder.