In an attempt to improve diagnosis of illnesses caused by parvovirus B19 and to discriminate primary from secondary infections, a protein-denaturing assay for avidity of parvovirus-specific IgG antibodies was developed. The assay used three types of purified recombinant antigens: a fusion protein containing the unique portion of the structural protein VP1, entire capsids made up of the major structural protein VP2 alone, or VP2 plus VP1. The avidity assays were evaluated by testing sequential acute-phase serum samples from 61 well-characterized patients (34 were followed > or = 6 months), sera from 38 controls with evidence of past infection, and sera from 388 seropositive patients studied for evidence of B19 infection during an epidemic. Parvovirus capsids consisting of VP2 alone yielded unusual IgG avidity and IgG antibody responses. Three different IgG avidity assays based on VP1 protein antigens were highly sensitive and specific and were considered suitable for identification of recent primary infections by human parvovirus B19.