The CD19-CR2 complex of B lymphocytes contains proteins that participate in two host-defense systems, the immune and complement systems. The ligand for the subunit of the immune system, CD19, is not known, but the complement receptor subunit, CR2 (CD21), binds activation fragments of the C3 component of the complement system and may mediate immunopotentiating effects of complement. A recombinant, soluble CR2 was prepared by fusing the C3-binding region of the receptor to immunoglobulin G1 (IgG1). The (CR2)2-IgG1 chimera competed with cellular CR2 for C3 binding and suppressed the antibody response to a T cell-dependent antigen when administered to mice at the time of immunization. This inhibitory effect of (CR2)2-IgG1 demonstrates the B cell-activating function of the CD19-CR2 complex and suggests a new method for humoral immunosuppression.