Interleukin 12 (IL-12) is a key cytokine in regulating type 1 or type 2 cytokine production and in determining the nature of immune responses. Our previous studies demonstrated that its production was significantly impaired in systemic lupus erythematosus (SLE) patients and this deficient IL-12 production was mainly mediated by excessive endogenous IL-10. The present study was designed to further reveal the relationships of in vitro IL-12 production with abnormalities of in vivo cytokine synthesis and disease activity in SLE. Experimental results showed that IL-12 production in vitro was inversely correlated with serum IL-10 level, anti-ds DNA antibody level and SLE disease activity index (DAI), but positively correlated with serum interferon gamma (IFN-gamma) level, with which serum IL-10 correlated negatively. Data also showed that serum IL-10 was significantly higher than that of controls and closely correlated with anti-ds DNA antibody level and SLEDAI. The study confirms that deficient IL-12 production in SLE patients is associated with in vivo abnormalities of cytokine production, especially with increased IL-10 production.