Placental steroid hormone production appears to be critical in maintaining pregnancy and possibly initiating parturition. Cytokines, produced by activated macrophages and decidua, are present during delivery, but their role in this process is not yet clear. To our knowledge, only one recent study, which used JEG-3 choriocarcinoma cells as an in vitro model, has evaluated the possibility that cytokines might affect placental steroidogenesis. Our current study reports observations on the effect of macrophage conditioned media (MCM, known to contain several cytokines) on the synthesis of progesterone and estradiol by term, normal, human placenta. Macrophage conditioned media significantly decreased progesterone (36 percent) and increased estradiol (76 percent) production by short-term placental organ cultures. These results suggest that macrophage secretory products might significantly alter placental steroidogenesis which could make them important factors in the physiology of parturition.