One hundred twenty strains of adenovirus types 1, 2, 5, and 6 (subgenus C), isolated at various times and places, were tested by neutralization and hemagglutination inhibition with two antisera of each type. Except for one type 5 strain they reacted largely type-specifically in both tests and neither qualitative nor quantitative differences were observed. Serologically intermediate strains were not found. The antigenic homogeneity is in contrast to the heterogeneity of the genomes of subgenus C adenoviruses.