The mode of genetic transmission of human phosphoribosylpyrophosphate synthetase (ribosephosphate pyrophosphokinase; ATP:D-ribose-5-phosphate pyrophosphotransferase; EC 2.7.6.1) was studied in fibroblasts cultured from members of a family with a structurally and electrophoretically altered phosphoribosylpyrophosphate synthetase that has increased activity per enzyme molecule. Enzyme activity in fibroblast lysates from the daughter of an affected male patient was intermediate to the activities in lysates from her father (and her affected paternal uncle) and from her mother and other normal individuals. Two bands of enzyme activity corresponding to normal and mutant phosphoribosylpyrophosphate synthetases were found in fibroblast lysates from the daughter after cellulose acetate strip electrophoresis. In contrast, only mutant enzyme was detectable in lysates derived from the male patients. Fibroblasts cloned from the daughter contained two phenotypically distinct (normal and mutant) populations of cells with respect to phosphoribosylpyrophosphate synthetase activity and electrophoretic mobility. These studies support assignment of the structural gene for human phosphoribosylpyrophosphate synthetase to the X-chromosome. No evidence for the presence of the normal enzyme was found in erythrocyte or lymphocyte lysates or in partially purified erythrocyte enzyme preparations from the heterozygous daughter, suggesting either nonrandom X-chromosome inactivation in precursors of these cells or selection against hematopoietic cells bearing the normal enzyme after random X-chromosome inactivation.