Ayruvedic medical practitioners in Sri Lanka use aqueous extracts of the mature leaves of Osbeckia aspera to treat liver disease. The extract has been shown to have hepatoprotective effects in vitro and in vivo, and to have inhibitory effects on the complement system and on in vitro phagocytosis by polymorphonuclear cells. The aim of this study was to investigate the effect of an aqueous extract of Osbeckia on lymphocyte proliferation stimulated by mitogens and antigen. In control peripheral blood mononuclear cells (PBMC), high concentrations of the Osbeckia extract were inhibitory to proliferation stimulated by phytohaemagglutinin (PHA) and tuberculin purified protein derivative (PPD). On stimulation by phorbol myristate acetate and ionomycin (PMA+I) the extract showed stimulation of proliferation at low concentrations (<10 microg/ml) with inhibition at higher concentrations. A similar inhibitory pattern on mitogen/antigen stimulation was seen with PBMC from patients with chronic hepatitis C virus (HCV) infection. These results suggest that the inhibitory agent(s) in the aqueous extract of Osbeckia may have an effect on antigen-presenting cell function. The combined hepatoprotective and immunosuppressive effects of the extract are more likely to be beneficial in acute hepatitis rather than chronic hepatitis viral infection.