The aim of the current paper was to determine the chorion's contribution to complement synthesis in the placenta and its regulation by cytokines. Biosynthetic labeling followed by immunoprecipitation with polyclonal antibodies was performed in chorionic tissue and chorion-derived cells. Eight complement proteins, factor B, C3, C1r, C1s, C1 inhibitor, factor H, C4 and C2 were detected in chorionic tissue and were secreted extracellularly. In chorion-derived cells, IL-1β stimulated factor B synthesis but had no effect on C1r, C1 inhibitor, C1s, factor H and C4. TNFα had no stimulative effect on any of the complement proteins tested. In contrast, both IL-1β and TNFα highly induced IL-6 secretion in chorion-derived cells, demonstrating the overall responsiveness of these cells to these stimuli. Interestingly, IFN-γ increased the synthesis of C1s, C1r, C1 inhibitor, C4 and factor H in chorion-derived cells. The fact that the latter two complement proteins have opposing effects on immune activation of the complement cascade demonstrates the complex balance required to both maintain an ability to ward off infections but simultaneously suppress the immune response to enable tolerance of the allograft fetus.