The physicochemical structure of the receptor for antibody (FcR) on B cells and its interrelationship with Ig and H-2 gene complex associated antigens were examined. FcR were found to be sensitive to treatment with phospholipase C and pronase, but resistant to neuraminidase, phospholipase A and chymotrypsin. They would therefore appear to be composed of phospholipoproteins. Several lines of evidence indicated that FcR and Ig receptors were discrete entities: thus, FcR (1) were resistant to chymotrypsin; (2) capped independently of Ig, as demonstrated by means of Fab fragments of anti-Ig, and (3) were closely associated with at least some la determinants, which are known to be distinct from Ig determinants. The relationship between FcR and H-2 gene complex associated antigens was confirmed by demonstrating inhibition of binding of aggregates by anti-la serum and vice versa. If, however, FcR were capped, anti-la serum applied under non-capping conditions was still found to bind diffusely to the great majority of B cells. Although this could be explained in part by the presence of residual FcR, some la determinants appeared to be distinct from FcR. The finding of residual FcR after capping with aggregates or immune complexes implied that FcR are a more integral part of the cell membrane than Ig receptors and could therefore act as proreceptors for the latter. Consistent with this was the demonstration of a significant polar distribution of Ig on B cells capped for FcR and then labelled under non-capping conditions with anti-Ig.