The ligand-receptor binding requirements for achieving full B cell activation through the membrane immunoglobulin (mIg) signaling pathway are relatively demanding, and mIg-antigen engagements which fall below these critical thresholds cause, at most, only the partial activation of B cells. In an effort to resolve new means of enhancing the efficacy of mIgM-mediated signal transduction, as well as to further understand the process by which mIgM-mediated signals are initiated, we have explored the mechanism for a previously reported synergy between certain mixtures of murine anti-IgM mAbs in eliciting human B cell DNA synthesis. We here report that striking synergy occurs when any of several relatively high affinity mAbs specific for diverse domains of mIgM are combined in culture with the relatively low affinity C mu 4-specific ligand, mAb IG6. Although B cell activation was dependent upon the bivalency, and hence mIgM cross-linking potential, of the high affinity ligand, low affinity mAb IG6 could enhance the activation process when present as a monovalent Fab' fragment. This did not appear due to F(ab')2 contamination or Fab' aggregation, since IG6 Fab' preparations were notably compromised in several other functions requiring ligand bivalency. Pulsing studies revealed that the C mu 4-specific ligand exhibits its functional effects only when stimulatory mIgM receptor cross-links are being formed by bivalent ligands, and that IG6 Fab' enhancement is most notable during the later interval of the prolonged mIgM signaling process that leads to S phase entry. A unique region of the membrane-proximal IgM domain may be important for Fab'-mediated enhancement, since Fab' fragments that bind with higher affinities to distinct sites on C mu 4 were not as effective at mediating this phenomenon. Several possibilities for the adjuvant effects of this C mu 4-specific Fab' on B cell responses triggered by mIgM crosslinking ligands are discussed, including the possibility that IG6 Fab' influences the potential for mIgM dimer formation or interactions of mIgM with other signal-transducing molecules.