Plasma fibronectin has a high affinity for denatured collagen (gelatin) and exerts an opsonic influence on phagocytosis of test colloids and clearance of tissue debris by macrophages. This study evaluated the effect of in vitro and in vivo interaction of gelatin with plasma on measurable bioassayable opsonic activity and immunoreactive fibronectin. Incubation of human, dog, sheep, and rat plasma with gelatin prior to in vitro assay decreased (P less than 0.05) the ability of plasma to augment particle uptake in the liver slice bioassay. Incubation of plasma with gelatin also decreased the concentration of fibronectin that could be detected by electroimmunoassay. Intravenous infusion of gelatin into rats, dogs, and sheep resulted in an acute depression in both bioassayable and immunoreactive opsonic observations suggest that deficits of opsonic fibronectin as documented in injured patients by bioassay and electroimmunoassay may not be exclusively related to actual depletion of fibronectin from blood but may be, in part, due to binding of fibronectin to blood-borne material post-trauma, i.e., collagenous tissue debris, whose presence in plasma may limit its detection by both assays.