Physiologically superactive phosphoribosylpyrophosphate (PRPP) synthetase, due to feedback resistance mutation, was found in a family with excessive purine production, gout and uric acid lithiasis. The superactivity of the mutant enzyme was manifest in the propositus' erythrocytes and cultured fibroblasts, in increased generation, content and metabolic availability of PRPP, leading in the fibroblasts to acceleration of the rate of purine synthesis de novo. One of the propositus' two siblings was similarly affected, but the propositus' father, his second brother and four sons, were all clinically and biochemically normal. The mother was clinically normal and normouricemic, but hyperuricosuric. Cultured fibroblasts from her skin exhibited variability in PRPP content and availability and in the rate of purine synthesis de novo. The mother's cultures were found to contain a mosaicism of two cell populations, one with normal and the other with mutant PRPP synthetase, indicating an X-linked pattern of inheritance of the PRPP synthetase abnormality in this gouty family.