Background: The presence of anti-parvovirus B19 (B19V) IgM against viral capsid proteins (VP1 and VP2) has long been used to detect recent infection. The utility of antibodies directed against B19V NS1 protein has received less attention as a serological indicator of recent infection, although anti-B19V NS1 IgG has been associated with persistent infection. Objectives: To elucidate the role of anti-B19V NS1 antibody detection in recent infection, full-length B19V NS1 was expressed and purified. The resultant antigen was used to develop both Western blot assays and microplate ELISA for the detection of NS1 antibodies. Study design: Serum specimens were obtained from individuals recently infected with B19V (children (n = 16), adults (n = 40)) and from 17 individuals with no evidence of recent B19V infection. All specimens were screened for anti-B19V NS1 IgG and IgM. Results: It was observed that 68.8% (11/16) of children recently infected with B19V were anti-B19V NSI IgG seropositive. Furthermore, 27.5% (11/40) anti-B19V VP2 IgM positive specimens also contained anti-B19V NS1 IgM when tested by ELISA, while no reactivity was observed following Western blot analysis, possibly due to the absence of conformational epitopes. Conclusions: Anti-B19V NS1 IgM detection may have utility in the confirmation of recent infection with B19V. (C) 2001 Elsevier Science B.V. All rights reserved.