The authors report a ten year study of active immunotherapy using BCG and irradiated allogeneic leukaemic cells in 200 patients. In acute lymphatic leukaemia, 57 out of 168 patients treated in this way remained in primary remission for 18 months to 10 years after active immunotherapy was begun, the relapse rate became low after 18 months and nil after 36 months. The results varied according to prognostic factors: the cytological type, active immunotherapy being above all effective in small cell (microlymphoblastic and prolymphocytic) types with a hope of cure in 50 to 60 p.cent of cases; malignant cellular volume; meningeal deposits. In microlymphoblastic forms the possibility of survival at the 5th year is greater than 90 p.cent. After relapse during active immunotherapy sensitivity to chemotherapy does not seem to be diminished. Trials of active immunotherapy in acute myeloid leukaemia are worthy of further pursuit. The results of active immunotherapy in leukaemic lymphosarcoma show that immunotherapy may be effective in preventing local recurrence, both of tumour as well as in the marrow. Four patients are in apparently complete remission for more than four years. On the basis of these results, trials of active immunotherapy for "residual disease" should be undertaken in the field of cancerology, going beyond the realm of leukaemias.