Interleukin-10 (IL-10) is known to be an anti-inflammatory cytokine which inhibits cell growth and cytokine production of both Th1 and Th2 cells. Using a human IL-10 ELISA kit we investigated whether serum IL-10 levels increased during the acute and convalescent stages in 45 children with rubella infections. Serum levels of IL-10 were markedly elevated in rubella patients during the acute stage, compared with those at the convalescent stage and those in healthy age-matched children (mean +/- SEM): 18.5 +/- 3.4 vs. 6.0 +/- 0.6 vs. 7.9 +/- 1.3 pg/ml. IL-10 levels determined 5 d after the onset of the disease had returned to the normal range. In patients with rubella, there were significant negative correlations between serum IL-10 levels and both rubella virus-specific IgM and IgG antibodies. These findings suggest that IL-10 may play a role in the pathogenesis of acute rubella infections.