Immune and inflammatory responses must be rightly regulated to maintain a homoeostatic balance between an effective immune response and tissue damage to the host. NO is a principal mediator of many of the cytokine-inducible macrophage activities during a normal cell-mediated immune response. STK, the murine homologue of the human RON receptor tyrosine kinase, is expressed on murine resident peritoneal macrophages. The ligand for STK, macrophage-stimulating protein (MSP), is a serum protein that is activated by members of the coagulation cascade in response to tissue damage. In addition to its potential to induce chemotaxis and phagocytosis of C3bi-coated erythrocytes, MSP has an inhibitory effect on the production of NO by activated peritoneal macrophages in vitro. Here we demonstrate that peritoneal macrophages from mice lacking STK produce elevated levels of NO in response to interferon (IFN)-gamma in a dose-dependent manner, without the need for a co-stimulus. However, production of pro-inflammatory cytokines by activated macrophages from stk -/- mice is unaltered. In vivo, stk -/- mice exhibit increased inflammation in an IFN-gamma-mediated delayed-type hypersensitivity reaction and increased susceptibility to lipopolysaccharide (LPS)-induced endotoxic shock. Furthermore, the levels of NO in the serum of mice injected with LPS are significantly higher than those in control littermates. Nevertheless, the serum levels of IFN-gamma and the intermediate cytokines generated by the inflammatory response, which have previously been shown to play a role in septicaemic shock, do not differ significantly from controls. These data suggest that the STK receptor suppresses NO production, therefore ameliorating the potentially tissue-damaging effects of a cell-mediated immune response, through negative regulation of the IFN-gamma signalling pathway.