The monoclonal antibody 4F2 recognizes a disulfide-linked ricin-binding glycoprotein complex (Mr congruent to 125,000) composed of a sialylated heavy subunit (Mr congruent to 85,000 on T cell lines) and an unsialylated light subunit (Mr congruent to 41,000). The antigen (T85,41) recognized by 4F2 on T cell lines is structurally distinct from the antigen (B93, 41) on B cell lines. The heavy subunits, but not the light subunits, from all T cell lines examined were uniformly smaller in size than the heavy subunits from several B cell lines. This reflects differences in carbohydrate rather than protein represent in B93,41 compared with T85,41, because both heavy subunits have a common unglycosylated form (p65) and a common partially glycosylated precursor form (p68). Among non-T, non-B hematopoietic cell lines, the monocytoid line U-937 expressed an antigen that resembles B93,41, whereas the erythroleukemic line K-562 expressed an antigen more similar to T85,41. 4F2 recognizes a protein determinant on the heavy subunit (with or without N-linked glycosylation) and also the unglycosylated heavy subunit retains the ability to associate with light subunit. The light subunit itself contains no detectable N-linked carbohydrate. Unlike the transferrin receptor, synthesis of the antigen recognized by 4F2 on the promyelocytic cell line HL-60 did not diminish upon dimethylsulfoxide-induced differentiation, and thus is not tightly correlated with cell proliferation.