It was shown that mouse antiserum against isologous thermically aggregated immunoglobulins MAAS injected repeatedly to the animals prolonged the survival of the skin allotransplant, induced Moloney's sarcoma diminished the latent period of the tumour development sharply increased the formation of the tumours which in the majority of cases, led to the death of mice, deranged the normal age barrier in fibrosarcoma induction, and sharply inhibited the intensity of Rauscher's leukemia development. This action of MAAS on the transplantation and antitumour immunity was of immunological nature.