The B1 cell surface molecule (CD20) is a 35‐kDa phosphoprotein expressed by B lineage cells during most stages of differentiation. Some monoclonal antibodies reactive with B1 induce activation while others, anti‐B1a, inhibit B lymphocyte function. To further determine the requirement of B1 molecule function in proliferation and differentiation the effects of anti‐B1a antibody binding on early cellular activation events were examined. Immunoglobulin secretion of lymphocyte cultures stimulated with pokeweed mitogen was maximally inhibited if the antibody (1‐10μg/ml) was added during the first 24 h of culture. However, even high antibody concentrations were unable to inhibit increases in free intracellular Ca²⁺ concentrations immediately following cross‐linkage of cell surface immunoglobulin, or inhibit cell enlargement and the expression of transferrin and interleukin 2 receptors. Antibody binding to B1 inhibited RNA synthesis (37‐80%) and progression through cell cycle following activation. In contrast, proliferation induced by phorbol myristate acetate was not inhibited by antibody binding to the B1 molecule. The findings that the earliest activation events and phorbol myristate acetate‐induced proliferation were not inhibited by antibody binding to B1 suggest that inhibition is due to the blocking of a step of the activation process required for cell cycle progression and differentiation, rather than blocking initial signal transduction across the membrane or providing a negative or suppressive signal.