A microneutralization procedure, using chicken kidney cell monolayers as an indicator system, was developed and applied to the serotyping of isolates characterized as avian adenoviruses. The method was determined to be reproducible, since coefficients of variation were low for 12 replicate titrations of homologous reagents of 9 prototype avian adenoviruses. Prototype reagents were specific according to results of reciprocal end point-neutralization tests and comparison of antigenic relatedness, using results obtained by previous researchers. Forty-two avian adenovirus isolates were classified into 6 serotypes by one-side end point-neutralization tests against antiserums made to 9 prototype avian adenoviruses. An additional 20 isolates were antigenically related to prototype viruses, but they could not be specifically types with the typing criteria. Different serotypes were isolated from birds having similar clinical diagnostic signs and lesions of disease.