CD21 is the receptor for C3dg and EBV. Several reports have shown that these CD21 ligands, and certain anti-CD21 mAb, trigger B cell activation, particularly when combined with Ag receptor ligation. However, the characteristics, biologic functions, and importance of this CD21-signaling pathway are unknown. We have used a model we recently developed to study B cell activation induced by complex particulate Ag, such as immune complexes and viruses, to begin to examine these questions. In the current studies, we incubated purified small resting B cells with 100-nm latex beads bearing various combinations of CD21 ligands and mAbs to CD19, CD35, and the Ag receptor. CD21, CD19, and CD35 have all been implicated in modulating membrane IgM initiated signaling. Beads coated with mAb to the C3dg/EBV-binding portion of CD21, but not mAb to other portions of the CD21 molecule, triggered B cell homotypic aggregation. Beads coated with the same CD21 ligands, although inactive alone, synergized with anti-IgM mAb in greatly increasing (20- to 180-fold) mRNA levels of the c-fos nuclear proto-oncogene. Signaling via CD21 was tyrosine kinase dependent. Levels of c-myc mRNA were not altered by CD21 ligands. Anti-CD19 and anti-CD35 mAb did not augment signaling via membrane IgM as assessed by changes in c-fos mRNA levels. These findings indicate that CD21 ligands binding to the C3dg/EBV-binding site of CD21 markedly augment B cell activation initiated by Ag receptor ligation via a selective, c-fos-dependent signaling pathway.