Sera from patients with symptoms of recent human parvovirus B19 (B19) infection were tested for B19-specific IgM in an immunofluorescence assay (IFA) using insect cells expressing B19 recombinant VP1 coat protein as an antigen. A highly significant correlation (P < 0.001) was found between titres obtained in the IgM IFA and the units obtained in an IgM antibody-capture RIA using plasma derived native B19 antigen. An IgG IFA using the recombinant antigen was performed on 57 sera and the antibody avidity determined. There was a highly significant correlation (P < 0.001) between the relative amounts of low avidity B19-specific IgG antibodies and time after onset of illness. This finding allows the detection of IgG to be used for diagnosing acute infection.