Immunological reactivity to hepatoma 22a was studied in newborn and adult mice pretreated with tumour antigens. Treatment of newborn mice enhanced the growth rate of hepatoma 22a. Analogous treatment of adult mice resulted in the immunization effect. Evidently this difference was connected with a possible change of immunological reactivity to the tumour in newborn mice. This is supported by the results of cell dynamics--a mediated immunity assessed by the macrophage migration inhibition test. Splenic cells of the pretreated adult mice possessed the capacity to depress the macrophage migration more intensively in comparison with those of the pretreated newborn mice.