Among affected members of a family (B. family) with excessive purine production and gout, activity of phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) is 2.5- to 3.0-fold higher than among normal people. The molecular basis for this increased enzyme activity was studied. Antibody to purified human phosphoribosylpyrophosphate synthetase of erythrocytes was obtained from immunized rabbits. Studies with the IgG fraction of this antiserum show the presence of normal quantities of immunoreactive enzyme, but 2.5- to 3.0-fold higher activity per molecule in affected members of the B. family. In addition, by use of a stain for phosphoribosylpyrophosphate synthetase activity, a difference in electrophoretic mobility was demonstrated on cellulose acetate gel between the partially purified enzyme from normal people and an affected member of the B. family. These studies suggest that the enzyme aberration responsible for purine overproduction and gout in the B. family results from a structurally altered enzyme with increased activity per molecule.