Latex particles coated with rat monoclonal antibodies directed against a canine parvovirus are agglutinated by the virus antigens. In this study, the reaction was quantitated by a technique which had been described as immunoassay by particles counting. The method is as sensitive as a radioimmunoassay with the same antibody, but has the advantages of simplicity and safety. It allows the detection of parvoviral antigens to 4 ng/ml. Incubation of these antigens with specific antibodies resulted in inhibition of the agglutination reaction. By this procedure the presence of antibodies against canine parvovirus can be detected to a concentration of 150 ng/ml. The system can be easily automated, thus increasing reproducibility. This latex agglutination technique can be performed as a slide test; and although this method is 4-5 times less sensitive, it could be useful in field studies, for the detection of the viral antigens and the corresponding antibodies.