The Cy34.1 monoclonal antibody specifically binds to the B-lymphocyte differentiation antigen CD22 on strains having the Lyb-8.2 alloantigen (e.g., A, BALB/c, CBA, C3H/He, C57BL, C57L, C58, SJL, SWR, but not AKR, DBA/1, DBA/2, NZB, PL). CD22 is expressed at high levels on mature peripheral B lymphocytes (follicular and marginal zone), B-1 cells (CD5+ B cells), and plasma cells. It is a member of the Ig gene superfamily and associates with the B-cell antigen receptor. Its sialic acid- binding immunoglobulin-like lectin (siglec) extracellular region mediates B-cell adhesion to ligands on endothelial cells in the bone marrow. Its intracellular domain is phosphorylated after cross-linking of antigen receptor or MHC class II antigen. It is involved in negative regulation of B-cell activation and protection from autoimmunity. B-cell proliferative responses to LPS or anti-mouse Ig ? chain are augmented in the presence of Cy34.1 mAb.