Lck is functionally coupled to CD5 in CLL B cells. Immunoblot analysis using anti-phosphotyrosine MoAb of Lck-specific immunoprecipitates from lysates of either purified B and T cells from a representative B-CLL patient or from control Jurkat T cells. The migration of Lck, as revealed by immunoblot with an anti-Lck MoAb of a control Jurkat T-cell lysate on the same gel, is indicated.