We have studied the catalytic, immunochemical, and electrophoretic properties of adenine phosphoribosyltransferase in hemolysates from 30 patients with a deficiency of this enzyme in six unrelated families. We have found that: 1) the level of adenine phosphoribosyltransferase enzyme activity and immunoreactive protein in the four homozygous deficient patients was less than 1% of control values; 2) adenine phosphoribosyltransferase enzyme activity was uniformly decreased to approximately 25% of normal in all 26 heterozygotes studied while the level of adenine phosphoribosyltransferase immunoreactive protein was consistent within each kindred but ranged in value from 22% to 112% of control; and 3) protein blot analysis revealed a single isoelectric form of the adenine phosphoribosyltransferase subunit in hemolysate from normal controls and from every heterozygote except for patient M.R. who exhibited both a normal and a more acidic adenine phosphoribosyltransferase subunit species. These studies provide the first evidence for the existence of a variety of different mutations in the structural gene for adenine phosphoribosyltransferase in patients exhibiting a deficiency of enzyme activity. We further conclude from our data that: 1) the variant enzymes are more labile in vivo and/or catalytically nonfunctional, and 2) heterozygotes express only 25% of normal enzyme activity because the normal and variant enzyme subunits form a hybrid dimer which is either more labile or less catalytically active than the normal adenine phosphoribosyltransferase dimer.