Peripheral blood lymphoid cells of 29 patients with acute lymphocytic leukaemia (ALL) at onset were studied for characterization of B and T membrane markers and phytohaemagglutinin responsiveness. 24 cases (83%) were classified as "null" cell ALL and 5 (17%) as T-cell ALL. No relationship could be found between cytological presentation and immunological classification. Moreover, no correlation has been demonstrated between clinical-immunological parameters and prognosis, indicating that in our series of patients, assessment of cell size and surface markers were not a reliable predictor of prognosis.