Skin fibroblast cultures were utilized to study the mode of inheritance of a mutant feedback-resistant phosphoribosylpyrophosphate synthetase in a gouty family with purine overproduction. Selective conditions were applied to allow the survival in culture of mutant cells only. Whereas in the male gouty propositus the cell culture was homogeneous for the mutant enzyme, in the cell culture from his nongouty mother two cell populations were demonstrated, one normal and the other mutant. The mosaicism in the mother is compatible with X-linkage of the enzyme. From this finding, together with the clinical and biochemical data available, it is concluded that in this family the enzyme mutation is transmitted in a X-linked recessive pattern.