We have determined the role of nitric oxide (NO)-mediated tumor cell killing in the treatment of an animal model of murine ovarian carcinoma grown in the peritoneum with a combination of cisplatin and cationic liposomes containing an expression vector for interferon-gamma (IFN-gamma). The approach was to determine whether the therapy was effective in mice homozygous for a disrupted inducible NO synthase (iNOS) allele; these mice were unable to produce NO in response to IFN-gamma. iNOS (-/-) mice treated with both cisplatin and liposomal IFN-gamma gene did not produce a significant amount of NO in ascites (12.1+/-4.5 microM), although they expressed a high level of IFN-gamma (9002+/-723 U/mL of ascitic fluid). As a result, mice died of tumors within 11-62 days. However, wild-type mice treated with both cisplatin and liposomal IFN-gamma gene produced a significant amount of NO in ascites (113.7+/-17.9 microM) with a high level of IFN-gamma gene expression (9350+/-1254 U/mL of ascitic fluid) and were free of tumors for at least 80 days. This result confirmed that NO was a direct mediator of IFN-gamma cytotoxicity.