The transmembrane protein Cbp/PAG (Csk binding protein/phospho-protein associated with glycosphingolipid-enriched microdomains) has a negative regulatory role in T cell activation as an adapter for C-terminal Src kinase, Csk. In T cells, membrane docking of Csk is promoted by binding to FynT-phosphorylated Cbp/PAG (pTyr317) to allow targeting of substrates residing in lipid rafts. Here, we investigate a potential parallel position for Cbp/PAG and the Src kinase Lyn in early B cell receptor signaling. Using normal and transformed B cells, we have compared signal profiles of BCR-triggered responses created by phospho-specific flow cytometry. In human normal B cells, our data show that reduced Cbp/PAG levels leads to enhanced and prolonged activation of proximal signaling mediators, while over-expression of the adapter in normal, EBV-transformed cells results in reduced calcium flux. Taken together, our findings support a negative regulatory function for Cbp/PAG in proximal BCR signaling in these cells.