Acute-phase serum from a patient with aplastic crisis provided sufficient human parvovirus B19 to make a monoclonal antibody against B19 and to develop antigen and immunoglobulin M (IgM) and IgG antibody detection enzyme-linked immunosorbent assays (ELISAs). The indirect capture antibody method was used for all three assays. Antigen was detected in 8 of 29 sera drawn within 2 days of onset of illness from patients with aplastic crisis. These sera had high titers of virus by electron microscopy and DNA hybridization and had no detectable B19 antibody. Antigen was not detected in serum specimens that had low titers of B19 DNA and had B19 antibody. With the IgM ELISA, we detected B19 IgM in over 85% of clinical cases of aplastic crisis and fifth disease and less than 2% of controls. The prevalence of B19 IgG antibodies increased with age. Approximately 2% of children less than 5 years of age and 49% of adults greater than 20 years of age had B19 IgG antibodies. The B19 antibody ELISAs are sensitive and specific tests to detect B19 infections.