The opsonic activity of plasma fibronectin is important in modulating the reticuloendothelial system (RES) phagocytic removal of a variety of endogenous and exogenous particulate material from the vascular compartment. Purification of plasma-opsonic fibronectin by affinity chromatography with gelatin-Sepharose revealed that although in vitro hepatic Kupffer cell phagocytosis was absolutely dependent upon the presence of fibronectin, the purified fibronectin evaluated in concentrations similar to that found in plasma (350-450 micrograms/ml) supported phagocytosis at a level two- to threefold less than that observed in whole plasma. In contrast, the combination of purified fibronectin with small aliquots of opsonically inactive fibronectin-free plasma restored normal opsonic activity as assessed by liver slice bioassay and enhanced fibronectin-mediated attachment of gelatinized particulate to isolated Kupffer cells in vitro. Evidence is presented in this study that there exists in plasma a macromolecular species that amplifies the opsonic activity of fibronectin in a dose-related manner. This amplification or cofactor activity is nondialysable and has a molecular weight greater than 12,000. Inactivation of the amplification activity present in affinity-absorbed plasma can be achieved by heating the fibronectin-free plasma at 60 degrees C for 20 min, supporting the hypothesis that the cofactor is a protein. The amplification response is dose related, suggesting that the mechanism of its action is stoichiometric rather than catalytic. Evidence is presented that partial purification of the cofactor can be achieved by (NH4)2SO4 precipitation at 4 degrees C. Purification of this cofactor will provide an opportunity to evaluate its role in the altered opsonic states known to exist after trauma, burn, and sepsis.