OBTAINED FROM HYPERIMMUNE SERUM OF HORSES IMMUNIZED WITH HUMAN THYMUS LYMPHOCYTES. REDUCES THE NUMBER OF CIRCULATING, THYMUS-DEPENDENT LYMPHOCYTES THAT FORM ROSETTES FROM SHEEP ERYTHROCYTES. ANTILYMPHOCYTIC EFFECT MAY BE DUE TO ALTERATION OF THE FUNCTION OF T-LYMPHOCYTES, WHICH ARE RESPONSIBLE, IN PART, FOR CELL-MEDIATED IMMUNITY.