We examined the biological function of a nonstructural regulatory protein, NS1, of human parvovirus B19. Because of the cytotoxic activity of NS1, human hematopoietic cell lines, K562, Raji, and THP-1, were established as transfectants which produce the viral NS1 protein upon induction by using bacterial lactose repressor/operator system. NS1 was significantly produced in the three transfectant cells in an inducer dose- and time-dependent manner. Surprisingly, these three transfectants secreted an inflammatory cytokine, interleukin-6 (IL-6), in response to induction. However, no production of other related cytokines, IL-1beta, IL-8, or tumor necrosis factor alpha, was seen. Moreover, NS1-primed IL-6 induction was transiently demonstrated in primary human endothelial cells. Analysis with luciferase reporter plasmids carrying IL-6 promoter mutant fragments demonstrated that NS1 effect is mediated by a NF-kappaB binding site in the IL-6 promoter region, strongly implying that NS1 functions as a trans-acting transcriptional activator on the IL-6 promoter. Our novel finding, IL-6 induction by NS1, supports the possible relationship between parvovirus B19 infection and polyclonal activation of B cells in rheumatoid arthritis and indicates that NS1 protein may play a significant role in the pathogenesis of some B19-associated diseases by modulating the expression of host cellular genes.