The sensitivity and application of the polymerase chain reaction (PCR) for the diagnosis of parvovirus B19 (B19) infection was investigated by simultaneously assaying a collection of 279 consecutively received samples for presence of anti-B19 IgM and IgG antibodies by Western blot and for B19 DNA by PCR and dot-blot hybridization (dot-blot); samples were sera from patients with suspected B19 infection. PCR and dot-blot detected B19 DNA in 9% (16/179) and 1% (2/179), respectively of Ab-positive samples (IgM+/IgG-, IgM+IgG+, IgM-IgG+), and in 28% (15/54) and 2% (1/54), respectively, of IgM+ samples. PCR also detected B19 DNA in 2% (2/100) of IgM-/IgG- samples, both of which had normal total IgG and IgM levels. PCR is of unique value because it permits diagnosis of B19 infection even in the absence of specific acute phase (IgM) and in the presence or absence of convalescent-phase (IgG) Ab.