Concanavalin A (Con A)section-binding proteins from mouse spleen leukocytes were characterised by two-dimensional electrophoresis of material precipitated, by Con A plus anti-Con A, from lysates of biosynthetically-labelled cells. Although most cell surface (iodinatable) proteins are known to bind Con A, some of the major Con A-binding proteins detected by immunoprecipitation, after a four-hour biosynthetic labelling period, are not iodinatable and are probably intracellular. Thus the major biosynthetically labelled Con A-binding species are: (i) a non-iodinatable, high molecular weight glycoprotein (C-145); (ii) intracellular precursors of secretory immunoglobulins (IgM and, probably, IgA); (iii) immature (not fully-sialylated) forms of H-2 D and K antigens; and (iv) Ia antigens. In the case of the H-2 antigens, (and possibly of other cell surface proteins) the selection of immature forms by Con A is not due to lack of biosynthetic labelling of mature products, but to preferential binding of Con A to incompletely glycosylated molecules.