Leukaemic cells from seventeen untreated acute lymphocytic leukaemia (A.L.L.) patients have been typed at presentation with heteroantisera prepared in rabbits by complement mediated cytotoxicity. Reactivity with antisera has suggested a preliminary grouping of patients, in this study, into Null, pre-T (post-thymic T-cell precursor) and T-cell subtypes. The leukaemic cells have also been classified with the established markers, E-rosette receptor, surface immunoglobulin and C3 receptor as well as total white cell count at presentation. Anti NALM-1 (Null-lymphoblastoid cell line) serum reacted with cells from all childhood A.L.L. patients tested but could be made specific for cALL antigen of the common Null form of A.L.L. by further absorption or dilution. Antisera to membrane fractions from cells of high white cell count Null-A.L.L. patients reacted with cells of these patients as well as T-A.L.L. patients. Anti MOLT-4 (adult T-A.L.L. derived lymphoblastoid cell line) serum reacted only with cells of T-A.L.L. patients.