Coexpression of two fibronectin receptors, VLA-4 and VLA-5, by immature human erythroblastic precursor cells. Human erythroblastic precursor cells adhere to fibronectin (Fn) but the exact nature of the receptors mediating this interaction has not been characterized. In this study, we report data showing that immature human erythroblasts express the integrins VLA-4 and VLA-5 and that both these molecules act as fibronectin receptors on these cells. We have recently demonstrated that adhesion to Fn of purified human CFU-E and their immediate progeny preproerythroblasts was inhibited by antibodies directed against the human fibronectin receptor (VLA-5). Here we have extended those results and characterized by immunoprecipitation with specific antibodies the integrins expressed on surface-labeled normal human immature erythroblasts. A polyclonal antibody recognizing the common VLA beta 1 subunit yielded two polypeptides of 120 and 160 kD. Our data further demonstrate that the polypeptide of 160 kD contains alpha subunits corresponding to both alpha 4 and alpha 5. Thus, erythroblast lysates prepared in 0.3% CHAPS and immunoprecipitated with antibodies which specifically recognize the alpha 4 subunit showed a heterodimer with peptides of 120 (beta 1) and 160 kD (alpha 4) and the additional peptides of 70 and 80 kD which usually coprecipitate with the alpha 4 chain. On the other hand, specific anti-alpha 5 antibodies immunoprecipitated an alpha 5/beta 1 complex with peptides of 120 and 160 kD which under reducing conditions migrated as a single band of 130 kD. Similar experiments performed with an erythroleukemic cell line (KU 812) showed that these cells also coexpress both the VLA-4 and VLA-5 members of the integrin family. Furthermore, monoclonal antibodies recognizing the VLA alpha 4 chain blocked the adhesion of immature erythroblasts to Fn-coated surfaces, thus demonstrating that, as VLA-5, VLA-4 is also a functional Fn receptor on these cells. 