Reference equine antisera to all 47 serotypes of human adenoviruses presently described have been prepared and evaluated by reciprocal neutralization and hemagglutination-inhibition tests. All tests were carried to endpoint dilutions a minimum of five times in each direction to give accurate values for homologous and heterologous antibody titers. Significant cross-reactions in the horse antisera were compared to similar data obtained from rabbit antisera. Using this analysis, major antigenic relationships exist among types 12-18-31 of subgenus A, types 7-11-14 and 34-35 of subgenus B, types 8-9-10, 10-19-37, 13-38-39, 15-22-42, 20-47, 24-32-33-46, and 29-45 of subgenus D, types 16-4 between subgenera B and E, and types 40-41 of subgenus F. Across all subgenera, types 8, 10, 13, 15, 17, 19, 26, 29, 39, 40, and 43 have antigenic moieties found most frequently in other types, averaging 12 heterologous reactions per type when summing both tests in both directions. Types 20, 30, 32, and 45 exhibit shared determinants slightly less often, with a mean of 8 heterologous reactions per type.