Leukaemia cells of 11 children with acute lymphatic leukaemias (ALL) and lymphoma cells of 4 children with lymphosarcoma (LS) were studied for the presence of T- and B-lymphocyte markers on their cell membranes. Spontaneous rosette formation (E-rosettes) of the malignant cells after adding sheep erythrocytes and the reaction with antithymocyteserum in cytotoxic test served as T-cell-markers and the surface immunoglobulins as B-cell-markers. The leukaemia cells of 5 ALL-patients with mediastinal tumours formed E-rosettes and reacted with the anti-thymocyte-serum. Six ALL-patients did not show these reactions. None of the ALL-patients had surface-immunoglobulins. Two of the 4 LS-children were E-rosette-positive, the cells of the other two LS-children contained surface immunoglobulins. There was a good correlation between the formation of E-rosettes and the reaction with the anti-thymocyte-serum. After absorption of the anti-thymocyte-serum with peripheral leukocytes it reacted with some of the E-positive lymphoblasts and thymocytes but not with normal peripheral T-lymphocytes. Anti-leukaemia-sera against the ALL-cells with T-cell markers of two patients after absorption with spleen cells did not react with peripheral lymphocytes but did effect lysis of thymocytes. Consequently, leukaemia cells which are E-rosette-positive possess a T-lymphocyte-associated antigen and a Thymus-associated antigen.