Plasma fibronectin (Fn) induces phagocytosis of C3b-opsonized sheep erythrocytes (EC3b) by human peripheral blood monocytes. However, Fn does not induce erythrophagocytosis of EC3b by human polymorphonuclear leukocytes (PMN), unless the PMN have been exposed to C5a or N-formyl- methionyl-leucyl-phenylalanine. Because of this difference, it is of great interest to examine Fn binding to cells that possess the capacity to differentiate into either granulocytes or monocytes. Hence, we have examined the consequences of Fn binding to the human myelomonocytic cell line, HL-60, both before and after in vitro differentiation of the HL-60, along a monocytoid or a granulocytoid pathway. Fn receptors were not found on undifferentiated HL-60, but several differentiating agents promoted the HL-60 binding of Fn-coated microspheres (Fn-ms). The peak of Fn-ms binding occurred four to five days after the induction of differentiation with dimethylsulfoxide (DMSO), and two days after induction by PMA. In addition, cells that differentiated along either the monocytoid or the granulocytoid pathway showed a marked increase in the phagocytosis of both IgG-coated erythrocytes (EA) and EC3b when they were exposed to Fn. Comparison of the effects of anti-Fn monoclonals on the binding of Fn-ms to the monocytes, PMN, and HL-60 showed that the same monoclonals block Fn-ms-binding and Fn-induced EC3b phagocytosis by all three cell types. Two monoclonal antibodies, M1/70 and A6F10, directed against membrane antigens on PMN and monocytes, inhibited Fn-ms binding. Both also blocked Fn-induced EC3b ingestion by these cells. However, neither antibody blocked Fn-ms binding or EC3b ingestion by differentiated HL-60. We conclude that differentiated HL-60 cells express functionally active Fn receptors, similar to monocytes and activated PMN, which, nonetheless, differ from normal cells in their association with the antigens recognized by M1/70 and A6F10.