The biliary glycoproteins (BGP or CD66a), a group of different splice variants of a single gene, are members of the carcinoembryonic antigen family and the immunoglobulin superfamily. Recently, we detected CD66a on IL‐2 activated lymphocytes. In this study we characterized the structure and the expression pattern of BGP on human lymphocytes and investigated its role in T cell activation. Lymphocytes express 2 of the 13 known splice variants, i.e. BGPa and BGPb, which are glycosylated in a lymphocyte‐specific manner. Both BGPa and BGPb have the long cytoplasmic tail, which contains two immunoreceptor tyrosine‐based inhibitory motif (ITIM)‐like motifs, but differ in their extracellular region containing 4 and 3 immunoglobulin‐like domains, respectively. On PBL BGP is expressed in small amounts only on B cells and Th cells. Stimulation with IL‐2 leads to a strong up‐regulation of BGP by these cells, and induces de novo BGP expression on γ δ T cells, CD8⁺ and CD56⁺ cells, but not on CD16⁺ lymphocytes. This up‐regulation of BGP seems to be part of the physiological process of T cell activation, since stimulation with anti‐CD3 mAb is sufficient to induce BGP up‐regulation. Based on the presence of the two ITIM‐like motifs, one may expect that BGP inhibits T cell activation, but surprisingly, engagement of BGP enhances the proliferation of anti‐CD3‐stimulated T cells.