The isoenzymes of hypoxanthine-guanine-phosphoribosyl transferase (HGPRT; E. C. 2.4.2.8) were studied by polyacrylamide gel disc electrophoresis in the erythrocytes of a family in which there was a partial deficiency of this X-linked enzyme. Hyperuricemic males, in whom HGPRT activity was 4% of normal, were found to have a variant enzyme which had altered kinetic and electrophoretic properties. In acrylamide gel, this variant migrated about 15% faster than the normal enzyme, and its K _m for hypoxanthine was twice that of the normal. The sister of two patients had 34% of normal activity in her erythrocytes and was thought to be a heterozygote. Electrophoresis of her hemolysate yielded profiles in which there were two zones of HGPRT activity. The more slowly migrating isoenzyme behaved electrophoretically like the normal isoenzyme. The faster-migrating isoenzyme had a mobility identical to that of the variant enzyme found in hemolysates from her hyperuricemic siblings. However, in her profile the activity of the variant enzyme was three times greater than that of the HGPRT found in the boys. This increased activity appears to be due to an interaction of the variant enzyme with the normal enzyme. Electrophoresis of a mixture of normal enzyme and the variant from a hyperuricemic male yielded a profile similar to that observed in this girl and a dramatic increase in the amount of activity in the variant zone.