<p>It is intended to provide a method of selecting an IL-2 production inhibitor and/or an immunocompetent cell proliferation inhibitor with little GATA-1 production inhibitory activity which comprises measuring the HDAC4 and/or HDAC8 inhibitory activity of a test substance; and a method of selecting an immunosuppressant with little platelet-reducing effect which comprises measuring the HDAC4 and/or HDAC8 inhibitory activity of a test HDAC inhibitor.</p>