SDS-polyacrylamide gel electrophoresis of anti-glucose-6-phosphate dehydrogenase immunoprecipitates from radiolabeled uterine tissue extracts previously revealed three proteins: A, B and C, which were tentatively identified as a 60-64 kDa precursor form, a 57 kDa predominant form, and 40-42 kDa nascent peptide form of the enzyme, respectively. A peptide-mapping technique was used to examine structural homologies among A, B and C. Following the labeling of uterine proteins with (35S)methionine, labeled proteins A, B and C were isolated by immunoprecipitation and electrophoresis. Each protein was individually co-digested with authentic, (3H)methionine-labeled glucose-6-phosphate dehydrogenase using papain, the resulting peptides were resolved by isoelectric focusing and the peptides from the two sources on each gel were compared using double-label counting methods. Proteins A, B and C had at least eight peptides in common, both proteins A and C had two additional peptides in common that were not present in protein B, and B protein had two peptides that were either absent or present in reduced amounts in digests of proteins A and C. The extensive structural homology and immunoreactivity of these proteins indicated that proteins A, B and C were all related to glucose-6-phosphate dehydrogenase.