CD5 is a transmembrane protein expressed on all T lineage cells and a subset of B cells. It is known that CD5 is physically associated with the T-cell receptor and B-cell receptor (BCR), inhibiting the signaling triggered by both of them. CD5 is also characteristic of B-chronic lymphocytic leukemia (B-CLL) B cells, although its implication in the development of this lymphoproliferative disorder has not been studied. In the present study, we examined the effect of CD5 in apoptosis, cell viability and global protein tyrosine phosphorylation mediated by BCR in B cells from B-CLL patients. As opposed to tonsil B cells, we did not observe an increase in the apoptotic or viability signals induced by anti-immunoglobulin M or SAC/interleukin-2 when CD5 was dissociated from BCR in leukemic cells of the majority of patients. We also observed that CD5 did not regulate the BCR-induced phosphotyrosine pattern in B-CLL B cells. These findings suggest that CD5 does not inhibit properly the BCR-mediated signaling in leukemic cells. This defect in inhibiting the BCR might contribute to the enhanced survival of B-CLL B cells.