CD44, also designated Pgp-1 or H-CAM, is a broadly distributed 85 kD protein. Amongst haematopoietic cells CD44 is expressed on B and T lymphocytes, monocytes and neutrophils. Other CD44-positive cell types include epithelial cells, glial cells, fibroblasts and myocytes. This broad distribution suggest a general role in cell-cell and cell-matrix adhesion. CD44 is involved in T cell activation and adhesion. Triggering of CD44 increases homotypic T cell aggregation mediated via LFA-1. CD44 is linked to the cytoskeleton. Furthermore CD44 has an accessory role in lymphocyte adhesion to high endothelial venules.