CD19 is a component of a cell surface receptor complex that regulates B lymphocyte responses to transmembrane signals including those generated through the B cell antigen receptor. Studies in mice which lack or overexpress CD19 show that changes in CD19 expression levels have significant effects on B cell development and function. Recent studies suggest that CD19 establishes a Src-family kinase activation loop that amplifies tyrosine phosphorylation of numerous downstream effector molecules including potentially positive and negative regulatory elements. These observations provide an understanding of how CD19 governs the molecular ordering and intensity of signals transduced through multiple B cell receptors.