Further purification of the lymphocyte inhibiting extract from the thymus.

A crude thymic extract, demonstrated to possess both immunosuppressive activity on T lymphocytes in vivo, and inhibitory action on DNA synthesis in T lymphocytes in vitro, has been purified by ultra-filtration and ion exchange chromatography procedures. The immunosuppressive activity was found to be associated with a high molecular weight fraction (greater than 50,000 daltons), devoid of immunosuppressive properties, a basic peptide has been isolated which retains the in vitro inhibitory activity on DNA synthesis.