We describe the identification of a monoclonal antibody that recognizes a determinant on the delta chain of mice of the Iga, allotype groups. The monoclonal Ig in soluble form induces allotype-specific proliferation by splenic B lymphocytes from normal animals of these haplotypes. Spleen cells from mice bearing the X-linked defect of CBA/N mice fail to respond, although they bear the determinant. Proliferation is independent of T lymphocytes. The data indicate a direct triggering function for sIgD.