To evaluate the effect of interleukin 10 (IL-10) on acute phase response, we determined serum levels of IL-10, interleukin 6 (IL-6), C-reactive protein (CRP), alpha-1-acid glycoprotein (AGP), alpha-1-anti-chymotrypsin (ACT) in 34 rheumatoid arthritis (RA) patients. IL-10 and IL-6 levels were evaluated using an enzyme-linked immunoassay (ELISA). CRP, AGP and ACT levels were measured using rocket immunoelectrophoresis. The results showed that IL-10 serum level was increased in RA patients as compared to controls (60.0 +/- 17.5 pg/ml vs. 7.2 +/- 1.9 pg/ml). IL-6 level was significantly elevated (87.3 +/- 32.7 pg/ml vs. 45 +/- 19 pg/ml, p < 0.05). CRP was significantly increased as compared to healthy controls (34 +/- 19 mg/1 vs. 3 +/- 2 mg/1, p < 0.05). AGP and ACT serum levels were increased in RA patients, but we did not find these changes to be statistically significant. A good negative correlation between IL-10 and IL-6 serum level was found (r = -0.73, p < 0.05). A positive significant correlation between IL-6 serum level and CRP (r = 0.62, p < 0.05), AGP (r = 0.74, p < 0.05) and ACT (r = 0.45, p < 0.05) was established. Moreover, a negative correlation between IL-10 and serum level of CRP (r = -0.76, p < 0.05), AGP (r = -0.60, p < 0.05) and ACT (r = -0.37, p < 0.05) was also shown. According to the data thus far obtained it seems that IL-10 decreases IL-6 production, and by that indirectly affects acute phase response, decreasing CRP, AGP and ACT synthesis.