Polyspecific monomeric immunoglobulin (Ig) isolated from either a commercial source (pooled, > 2000 donors), or an autologous donor was capable of inhibiting both B cell proliferation, induced by T dependent mitogens or T cell factors and B cell differentiation, induced by similar stimuli. These effects appear to be directed at the B cell itself since inhibition of differentiation is detectable when monomeric Ig is added to cultures of B cell lines in the presence of B cell differentiation factor (BCDF). The inhibition of B cell differentiation does not appear to relate to inhibition of B cell proliferation, as no detectable change is seen in thymidine incorporation or cell number in these cultures. Furthermore, the effect of monomeric lg appears to relate to an early event in B cell differentiation, as there is no effect of IgSRK on spontaneously secreting B cell lines and maturation to cytoplasmic Ig containing cells is markedly impaired. Therefore, monomeric Ig secreted by B cells may serve as an immunoregulator of further Ig secretion.