The vast majority of human lymphoid neoplasms examined to date have been associated with a proliferation of bone marrow-dependent (B) lymphocytes. In an effort to delineate human tumors of T-cell (thymusdependent) lineage, use was made of the peripheral blood leukocytes of sixteen subjects with various forms of mycosis fungoides. The abnormal cells in the circulation of these patients are morphologically identical to those that infiltrate their nodes and skin. On electron microscopy, such neoplastic lymphocytes (S6zary cells) had "cerebriform" nuclei and an abundance of cytoplasmic fibrils not described heretofore. S6zary cells were nonadherent and nonphagocytic and usually responded to stimulation with phytohemagglutinin, refuting earlier suggestions that the cells represent monocytes or histiocytes. In contrast to chronic lymphocytic leukemia lymphocytes, the Szary cells lacked surface immunoglobulin and receptors for complement. Ultrastructural analysis identified S6zary cells in the center of directly formed rosettes (E-rosettes) characterizing the behavior of T lymphocytes in this test.Though some S6zary cells lacked both T and B cell-surface properties, in general, these observations support the view that the S6zary cell is a neoplastic variant of a thymusderived lymphocyte.