CD44 represents a family of type I transmembrane glycoproteins expressed on many cell types such as B cells, monocytes, macrophages, and variable subsets of thymocytes and peripheral T cells. CD44 primarily functions as an adhesion molecule. It has been implicated in various processes, such as hematopoiesis, lymphocyte homing, leukocyte activation, tumor metastasis and development. The KM201 monoclonal antibody reacts with an epitope very close to the hyaluronate binding domain on the CD44 molecule. KM201 can inhibit hyaluronate-dependent cell aggregation, prevent lympho-hemopoiesis in both Dexter and Whitlock- Witte Cultures, prevent the earliest intrathymic precursors from homing to the thymus, and costimulate the activation of freshly purified splenic CD4+ T cells.1-6