The effects of T helper lymphocyte and Ia" cell depletion were examined for their ability to independently and synergistically achieve prolongation of xenogeneig (rat-to-mouse) islet transplants, Recipient mice were depleted of T helper lymphocytes by short-term treatment with the anti-L3T4 monoclonal antibody GK1, 5. Donor rat islets were treated prior to transplantation with a concentration of anti-Ia immunotoxin (13, 4XRT) that selectively depleted Ia" cells within the islets while leaving functional insulin-secreting (8-cells unaffected. Anti-L3T4 treatment alone allowed transplants to be prolonged compared with untreated controls; however, all such treated miçę rejeted their xenogenei transplant within 22 days, Although 13.4 x RT treatment of