A cell surface molecule that is expressed specifically in thymocytes, lymphocytes activated by ConA-stimulation, and peripheral blood lymphocytes. This molecule is involved in signal transmission of the secondary signal (costimulatory signal) essential for the activation of lymphocytes such as T cells and regulates functions of activated lymphocytes such as activated T cells. Disclosed are an antibody or a portion thereof, which binds to a polypeptide of the cell surface molecule, a polypeptide fragment thereof, or a fusion polypeptide comprising the fragment; a cell secreting the antibody or its portion; a pharmaceutical composition comprising the antibody; and methods of using the compositions for therapeutic, diagnostic and/or experimental purpose.