Fasciola hepatica infection has been shown to suppress sheep lymphocyte proliferation in vitro and this is at least partially attributable to excretory/secretory products (ES) released by F. hepatica parasites. We identified a suppressive component in ES by analysing the effect of ES fractions, separated by gel filtration, on the proliferation of sheep T cells in vitro. A major proportion of the suppressive activity in ES was shown to coelute with the cathepsin L proteases: E64, a cysteine protease inhibitor, blocked the suppressive activity of cathepsin L. In order to identify possible mechanisms by which cathepsin L could suppress T cell proliferation, the effect of ES and F. hepatica recombinant cathepsin L (rFhCatL) on the expression of 22 different sheep T cell surface markers was analysed by flow cytometry. Incubation of sheep T cells with ES or two rFhCatL significantly reduced surface CD4 expression and this effect was prevented in the presence of E64. In similar experiments with human T lymphocytes, ES and rFhCatL were shown to down regulate surface CD4 expression. These results show that F. hepatica cathepsin L both suppresses sheep T cell proliferation and reduces surface CD4 expression on both human and ovine T cells.