Immunohematologic studies on cells from a patient with the clinicopathologic syndrome of hairy-cell leukemia showed that the neoplastic cells had receptors for sheep erythrocytes and therefore had human T-lymphocyte characteristics. The leukemic cells did not have the membrane receptors or immunoglobulin markers of B lymphocytes or monocytes nor did they synthesize immunoglobulin. A lymphoid cell line established in vitro from the cells had the same T-lymphocyte characteristics. The lymphoid cell line is positive for tartrateresistant acid phosphatase, forms rosettes with untreated sheep erythrocytes, and reacts with an anti-T-lymphocyte antiserum. Thus the syndrome of hairy-cell leukemia may occasionally result from the neoplastic proliferation of T-lymphocytes as well as from the more usual B-lymphocyte form. This situation is analogous to that described previously in chronic lymphocytic leukemia and other lymphoproliferative disorders.