Spontaneous rosette formation with sheep erythrocytes (SRBC) was studied in the peripheral blood and bone marrow lymphoid cells from a patient whose leukemic cells appeared to be T-lymphocytes. Simultaneous morphological examination of the peripheral blood white cells indicated that they consisted of 21% lymphoblast; 26% prolymphocytes and 48% mature lymphocytes. The distribution of bone marrow cells within the cell cycle was determined by flow microfluorometry and 7 hours after treatment with vincristine consisted of 69% in G1, 21% in S, and 9% in mitosis. Since virtually all the cells both in marrow and blood formed rosettes with SRBC this implies that the expression of this T cell marker is independent both of the morphological appearance of these cells and their position within the cell cycle.