Murine CD72.1 (Lyb-2.1), a type II integral membrane glycoprotein and a member of the C-lectin family of cell surface receptors, is a differentiation antigen of B cells and is found in mouse strains expressing the Lyb-2.1 allotype. CD72.1 is the ligand of CD5 which is distributed on all T cells and a small number of B cells. The 10.1.D2 monoclonal antibody blocks binding of CD5 to CD72 on the B cell surface which leads to inhibition of the positive signal resulting from CD5/CD72 pairing. However, 10.1.D2 can itself trigger a positive signal by binding CD72.