In the absence of serum inhibitor human parvovirus B19 agglutinates primate red cells. Agglutination of baboon erythrocytes by inhibitor free baculovirus expressed VP1/VP2 proteins of B19 virus was enhanced by low pH. To avoid interference from inhibitor(s) present in serum specimens microtitre tests with an antibody capture configuration were designed and optimised. Antigen binding was demonstrated by the adherence of baboon erythrocytes. These tests (MACHAT, GACHAT) detected IgM or IgG anti-B19 in 1032 serum specimens with an overall sensitivity of 99%, specificity of 98.2% and positive predictive value of 96.8% with reference to the radioimmunoassays currently in diagnostic use. MACHAT and GACHAT are straightforward, and well suited to large scale research and seroepidemiological investigations. They are without the risks from infectious antigen and radiation associated with existing radioimmunoassays and allow the phenomenon of B19 virus haemagglutination to be put to practical advantage.