In different treatment groups of children with acute lymphatic leukaemia and undifferentiated leukaemia, the lymphocyte count and a number of immunologic parameters were examined during the various stages of treatment. These parameters were related to the type of chemotherapy as well as the stages of treatment. This paper deals with the values of lymphocyte counts, of gamma globulin, IgG, IgM and IgA and the results of the DNCB skin tests, as a parameter for cellular immunity. IgG determined by acetate sheet electrophoresis showed no significnat change, while its significant reduction was shown during the periods of prophylactic reinduction with vincristine in the Pinkel VII scheme, The IgM level was significantly raised in the initial stage before treatment and in all periods of treatment both in the Pinkel VII group and in an earlier therapeutic group treated with methotrexate every two weeks and vincristine pulses every six weeks. IgA was raised in both groups before treatment but significantly reduced during the phases of maintenance and reinduction. About 50% of the P VII cases had negative DNCB skin tests during maintenance therapy, while all long-term survivors in the earlier methotrexate group had a positive skin test.