A panel of monoclonal antibodies (MoAbs) against cell surface proteins of early BFUe progeny was characterized. Five of these antibodies (Abs) reacted with normal erythroid, but not myeloid, bone marrow cells. Each of the five antibodies, typified by Ab 69.20, immunoprecipitated a dimeric complex of 185,000, which is composed of two identical disulfide-bonded subunits. This antigen had affinity for transferrin, and was essentially identical in biochemical characteristics to transferrin receptors precipitated with the well-characterized MoAbs OKT9 and 5E9. However, this form of transferrin receptor lacked both the OKT9 and 5E9 antigenic determinants and, moreover, the 69.20 epitope was absent from the conventional transferrin receptor, as defined by Abs OKT9 and 5E9. Modulation experiments demonstrated that both 69.20 and OKT9 modulated large, virtually independent populations of transferrin receptors. Both forms of transferrin receptor appeared to be derived from the product of a single gene, but the form defined by MoAb 69.20 apparently predominates in cells of the erythroid lineage and some transformed cell types that manifest a special requirement for iron. These data suggest that cells with a high iron requirement synthesize two forms of transferrin receptor, possibly by means of differential mRNA splicing or by posttranslational modification of the transferrin receptor.