The involvement of plasma fibronectin in phagocytosis of bacteria was investigated by testing the binding of fibronectin to several species of bacteria and by evaluating the ability of fibronectin to promote binding and endocytosis of two species of these bacteria by phagocytic cells. Fibronectin binds non-covalently to Gram-positive and Gram-negative bacteria and to yeast but did not appear to be necessary or sufficient for uptake of Staphylococcus aureus and Salmonella typhimurium by several different phagocytic cell types.