Recombinant human interferon-gamma (r-hu-IFN-gamma) has been found to exert an antitumor action in vivo in early stages of human malignant mesothelioma, and an antiproliferative effect in vitro. In order to study the mechanisms of cytostasis in mesothelioma cells, we examined two IFN-gamma-controlled metabolic pathways known to mediate growth arrest in various cell types, measuring production of the antiproliferative compound nitric oxide (NO) and degradation of tryptophan in nine human mesothelioma cell lines (HMCLs) displaying different sensitivities to the antiproliferative effect of r-hu-IFN-gamma. Two rat mesothelioma cell lines were also studied. IFN-gamma receptor was present and functional in HMCLs, regardless of their sensitivity to the growth-inhibitory effect of r-hu-IFN-gamma. However, no NO synthase activity or the resulting antiproliferative molecule NO were induced in HMCLs treated either with r-hu-IFN-gamma alone or with a combination of r-hu-IFN-gamma and other cytokines, and/or with lipopolysaccharide (LPS). In responsive HMCLs, r-hu-IFN-gamma induced strong indoleamine-2,3-dioxygenase (IDO) activity, which causes rapid degradation of tryptophan; however, the correlation between r-hu-IFN-gamma-mediated growth arrest and IDO induction was not absolute. In rat mesothelioma cells, NO synthase was induced in response to murine IFN-gamma + interleukin-1beta (IL-1beta) treatment, and played a role in the cytokine-mediated antiproliferative activity. However, NO production did not seem to be the unique antiproliferative mechanism induced by cytokines in these cells. Our results indicate that two classical pathways accounting for some of the cytostatic effects of IFN-gamma in rodent cells are not efficient in human mesothelioma cells, and suggest that cytokine-induced growth inhibition is mediated by a different pathway in HMCLs.