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 (Ly-1) which is distributed on all T cells and a small number of B cells.1,2 The 10.1.D.2 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.1 However, 10.1.D.2 can itself trigger a positive signal by binding CD72.1-5