Human chorionic gonadotropin (hCG) is a placental hormone and is involved in maintenance of the corpus luteum during pregnancy. In the present study, effect of hCG on nitiric ox ide (NO) generation from peritoneal macrophage was examined. hCG ahd no effect on NO generation by itself, whereas recombinant interferon- ${gamma}$ (rIFN-${gamma}$) alone had modest activity. When hCG was used in combination with rIFN-${gamma}$, there was a marked cooperative induction of NO generation in a dose-dependent manner. The optimal effect of hCG on NO generation was shown at 6 hr after treatment with rIFN-${gamma}$. Furthermore, northern blot analysis of showed that hCG increased the expression of inducible NO synthase(iNOS) gene. These results suggest that hCG induces NO generation from macrophages by increasing the expression of iNOS gene.