An antiserum against human fetal thymus was prepared in rabbits and absorbed with liver cells from the thymus donor and lymphocytes from a patient with chronic lymphocytic leukemia (CLL). The antiserum reacted with 80%-95% of the peripheral small lymphocytes from 12 normal controls but with only 1%-11% of cells from 12 patients with CLL. The antithymocyte antiserum demonstrates a cell surface antigen present on the majority of normal human lymphocytes, but absent from the neoplastic CLL cell. This finding is consistent with the view that the CLL lymphocyte is devoid of T-cell antigen and with other evidence that this lymphocyte closely resembles the B-cell of experimental animals. It appears likely that leukemic cells will prove useful in preparing an antihuman thymocyte antiserum that reacts exclusively with $$Word$$ cells by removing B-cell reactivity.