Fibronectin is a high-molecular-weight glycoprotein found as a soluble dimer in plasma and as an insoluble multimer in tissues. It has been proposed that plasma fibronectin facilitates phagocytic removal of lysed cells and damaged tissues. Fibronectin binds avidly to several species of gram-positive bacteria and enhances staphylococcal and streptococcal attachment to cultured cells. Determination of whether fibronectin will enhance the bactericidal activity of monocytes and macrophages has not been reported. The bactericidal activity of freshly isolated monocytes, cultured monocytes, or lymphokine-activated macrophages was tested in the presence of either dimeric or multimeric fibronectin. Freshly isolated monocytes and lymphokine-activated macrophages killed Staphylococcus aureus effectively in the absence of fibronectin or whole serum. In contrast, monocytes cultured for 7 to 10 days had diminished staphylocidal capacity. When the monocytes were cultured with either dimeric or multimeric fibronectin, however, bactericidal capacity was maintained. Thus, although fibronectin did not enhance the bactericidal activity of mononuclear phagocytes, both multimeric and dimeric fibronectin were effective at maintaining the bactericidal capacity.