The studies presented here examine the effect of a monoclonal rat antibody, LR-2, on the proliferation of growth-inhibitable murine B lymphoma lines. Like anti-immunoglobulin, mAb LR-2 is capable of inducing growth arrest in these cells, implying that ligation of the LR-2 cell surface molecule by mAb LR-2 transduces an inhibitory signal. The growth inhibition produced by LR-2 can, however, be clearly distinguished from that produced by anti-Ig reagents. Our findings show that mAb LR-2 does not react with membrane Ig. Significantly, the LR-2 antigen could be induced to comodulate with membrane Ig. The data thus indicate that mAb LR-2 reacts with a mIg-associated signal-transducing molecule on the B cell surface and may constitute a component of the Ig receptor complex. The role of this molecule in the regulation of the proliferation and maturation of normal B cells is currently being investigated.