Fu-ling is one of the most widely used Chinese herbal medicines. In this study, we investigated the regulatory effect of fu-ling on interleukin-10 (IL-10) production in vivo. Mice were i.p. administered 0.1 mg to 1.0 mg fu-ling per gram body weight daily for three days. The spleen cells were isolated and assayed for both IL-10 and immunoglobulin (Ig) production. Results indicated that the mice treated with fu-ling had significantly increased spleen cell ability to secrete IL-10. Spleen cells isolated from the mice injected with either 0.1 mg or 1.0 mg fu-ling per gram body weight also showed an increase in IL-10 mRNA expression. As IL-10 is a potent differentiation factor of B-lymphocytes, the possible effect of fu-ling on Ig production was also studied. Results indicated that fu-ling significantly induced an increase in IgG and IgA secretion by spleen cells but showed no effect on IgM secretion. Thus, fu-ling may affect the function of B-lymphocytes via stimulating IL-10 production.