Human thymus [T] cells and blasts from some patients with acute lymphoblastic leukemia (ALL) express similar cell surface properties. This suggests that this type of ALL originates in the thymus or that these properties of ALL blasts reflect changes occurring during blastogenesis. To test these possibilities, it was determined whether mitogen-induced blasts would express [sheep] erythrocyte (E) receptors and thymus antigens found on blasts from E+ALL and normal human thymocytes. E-rosettes of blood T-lymphocytes dissociated at 37.degree. C. Rosettes formed by E+ALL blasts, human thymocytes and pokeweed mitogen stimulated blasts were stable at this temperature. Blasts that formed stable rosettes did not have cytoplasmic Ig [immunoglobulin] indicating that they were T-lymphoblasts. Immunofluorescence and a radiolabeled antibody assay demonstrated a thymus antigen(s) that was present on the membrane of E+ALL blasts and normal thymocytes, but not on normal blood T-lymphocytes [TL-like antigen(s)]. This antigen was not expressed on blasts induced by mitogens. The finding that mitogen-induced blasts form temperature-stable rosettes, but lack TL-like antigen(s), indicates that this antigen is not required for the expression of E-receptors stable at 37.degree. C. The results support the concept that E+ALL results from malignant transformation of thymus cells.