We have developed an indirect microhemagglutination test (IMHA) for detecting antibodies to Borrelia burgdorferi using formalin-fixed chicken erythrocytes sensitized with a sonicated extract of the bacteria. The specificity of IMHA for Lyme disease was compared with that of a conventional indirect immunoperoxidase technique (IP) using sera from the patients with syphilis, randomly-chosen outpatients unrelated to Lyme disease, and from normal individuals. The IMHA was more specific than IP, giving little cross-reaction with syphilis. The IMHA also gave little background reactions with the sera of individuals unrelated to Lyme disease or syphilis. Therefore, the IMHA could be a useful test for the serodiagnosis of Lyme disease.