Cells from patients with acute lymphocytic leukemia were studied for cell surface markers and B-cel alloantigens. Three cell types were defined. Those spontaneously rosetting with erythrocytes and not prossessing surface immunoglobulin or complement receptors were considered T-cells. Cells not showing rosette formation, surface immunoglobulin, or complement receptors were considered null cells and could be divided into two categories based on their ability to stimulate in mixed lymphocyte reaction. B-cell alloantigens were found randomly distributed on all cell types. The results suggested that genetic information for human B-cell antigens was present in all cell types and that other cell surface markers may reflect a stage of differentiation of the leukemia cell type.