CD72, a B cell surface protein of the C-type lectin superfamily, recruits the tyrosine phosphatase SHP-1 through its ITIM motif(s). Using CD72-deficient (CD72-/-) mice, we demonstrate that CD72 is a nonredundant regulator of B cell development. In the bone marrow of CD72-/- mice, there was a reduction in the number of mature recirculating B cells and an accumulation of pre-B cells. In the periphery of CD72-/- mice, there were fewer mature B-2 cells and more B-1 cells. In addition, CD72 is a negative regulator of B cell activation, as CD72-/- B cells were hyperproliferative in response to various stimuli and showed enhanced kinetics in their intracellular Ca2+ response following IgM cross-linking.