The hematopoietic cell recognition sites of human fibronectin (FN) are the Arg-Gly-Asp-Ser (RGDS) sequence recognized by widely distributed integrin receptor alpha 5 beta 1 and the type III connecting segment (III CS) containing two cell-binding sites, designated CS1 and CS5, that are recognized by the alpha 4 beta 1 receptor. The C-terminal heparin-binding domain of FN (Hep II) has recently been demonstrated to support adhesion of alpha 4 beta 1-dependent melanoma cells [A. P. Mould and M. J. Humphries (1991) EMBO J. 10, 4089-4095]. Previously we demonstrated that this region of FN mediated binding of FN to HL-60 cells (acute promyelocytic leukemia cell line) by direct interaction independently of RGD and CS1 [H. Fujita et al., (1995) Exp. Cell Res. 217, 484-488]. In this study we have characterized a novel site in the Hep II region for binding to HL-60 cells. alpha 4 beta 1 and alpha 5 beta 1 were expressed on HL-60 cells, while alpha 2 beta 1 and alpha 3 beta 1 were not present, as shown by flow cytometry using monoclonal antibodies specific for the different integrins. Anti-alpha 4 beta 1 (P4C2) and anti-beta 1 (JB1a) antibodies inhibited binding of a 29-kDa dispase-digestive fragment of FN to HL-60 cells. This fragment contains the C-terminal heparin-binding domain of FN but lacks CS1 and CS5. Only the peptide representing the sequence from Val1866 to Arg1880, designated E1, inhibited the binding of the 29-kDa fragment to HL-60 cells. The active region of this peptide was a sequence of Thr-Asp-Ile-Asp-Ala-Pro-Ser (TAI-DAPS), which is homologous to Leu-Asp-Val-Pro-Ser (LDVPS) derived from the active site of CS1. Furthermore, labeled E1 peptide directly bound to HL-60 cells. The anti-alpha 4 beta 1 antibody (P4C2) inhibited this interaction. These results indicate that the site of binding to hematopoietic cells is present in the Hep II region of FN and the definition of the chemical structure of FN clarifies a fundamental mechanism of cell invasion of the extracellular matrix.