Human alpha-thrombin, in addition to its procoagulant activity, is a mitogen for fibroblasts and endothelial cells and a chemotactic agent for monocytes. To further understand the complex physiological functions of thrombin, we investigated whether thrombin has any immunoregulatory function with regard to T cell activation. Using highly purified human alpha-thrombin and peripheral blood mononuclear cells (PBMC), we investigated whether thrombin has any effect on cytokine production and/or proliferation induced by different T cell stimuli. At physiological concentrations (1-10 micrograms/ml, 30-300 nM), thrombin significantly enhances T cell proliferation in response to mitogens, superantigens, alloantigens, and anti-CD3 stimulation. Enhanced proliferation was associated with increased IL2 and IL6 production and with an increase in the number of IL2r+ (CD25)-bearing T cells. Thrombin alone was not mitogenic nor did it induce IL2 production or increase the number of IL2r+ T cells. However, PBMC exposed to thrombin alone produced high levels of IL6. Thrombin also enhanced IL2-induced proliferation of murine and human IL2-dependent cell lines. These results suggest that thrombin may play an important role in regulating cell-mediated immunity.