SummaryA rise in antibodies to types 1-4 of adenoviruses during illness can be detected in human serum by employing hemagglutination technics. Although the original method described does not measure an increase in antibodies directed against a single type of virus, type-specific antibodies can be titrated if serum is mixed with a pool of heterologous viruses before tanned rbc treated with homologous virus are added. This modification measured type-specific antibodies solely in 4 of the 8 sera tested. The antigen adsorbed to tannic acid-treated rbc is probably not the infectious viral particle and is separable from it by high speed centrifugation.