B ecause of their paternal antigens, the fetus and placenta may be considered an allograft in the maternal host. Understanding the mechanisms which prevent maternal immunological rejection of the fetus remains a fundamental unsolved problem in immunology. We have previously reported that macrophages are inhibited by maternal decidual stromal cells residing at the maternal-fetal interface. In view of the central role of macrophages in cell-mediated immunity, this inhibition may contribute to preventing maternal antifetal responses. We now report that it was the solid phase signals embedded in the extracellular matrix (ECM) made by decidual cells which are responsible for inhibiting macrophage-mediated lysis of TNF-alpha-resistant P815 mastocytoma cells. The latter macrophage function is acquired after stimulation by interferon gamma and endotoxin. All these macrophage functions were also inhibited by ECM isolated from the Engelberth-Holm-Swarme (EHS) tumor. This tumor ECM has a similar biochemical composition to decidual ECM. This ECM inhibited the effector, as opposed to the stimulator, phase of macrophage-mediated tumor lysis. Laminin, type IV collagen, and heparan sulfate proteoglycans, the major known components of decidual and EHS ECMs, did not inhibit the above macrophage functions. Altogether these data indicate that macrophages were inhibited by solid phase signals embedded in decidual and EHS ECMs. Whether the solid phase signals in these two ECMs are biochemically identical remains to be determined. To our knowledge, such signals are a novel pathway of inhibiting macrophage functions which may be important in understanding the maternal-fetal immunologic relationship, and the pathogenesis of perinatal infections. Furthermore, the ability of EHS tumor ECM to inhibit macrophage functions may indicate that some tumors may defend themselves against host macrophage responses using solid phase signals. This may be important in understanding some host-tumor relationships.