Previously we reported that activated T lymphocytes express a heparanase enzyme that degrades the heparan sulfate moiety of the proteoglycan of the extracellular matrix (ECM). Expression of the heparanase enzyme was found to be associated with the ability of activated T lymphocytes to penetrate blood vessel walls and accumulate in target organs. We recently found that relatively low doses of heparin administered to mice or rats inhibited T cell-mediated immune reactions. In the present study we investigated the effects in vitro and in vivo of the heparanase inhibitor, heparin, on the expression of T lymphocyte heparanase and on the ability of T lymphocytes to mediate a delayed-type hypersensitivity (DTH) reaction. We found that heparanase was induced by immunizing mice with antigen in vivo or by activating T lymphocytes with concanavalin A in vitro. Relatively low doses of heparin administered once daily in vivo (5 micrograms) or present in vitro (0.1 microgram/ml) inhibited the expression of heparanase induced by immunization or by concanavalin A incubation. Higher or lower doses of heparin did not have these effects. The same doses of heparin that inhibited expression of heparanase also inhibited the ability of the lymph node cells to migrate to a site of antigen and adoptively produce a DTH reaction. These findings suggest that modulation of cell-mediated immune reactions may be achieved by relatively low doses of heparin which inhibit expression of T lymphocyte heparanase.