Immunization of adult animals with the Ehrlich ascytic cancer cells homogenate three months prior to an experiment, did not affect either tumor transplantation or the progress of cancerogenesis induced by injection of 20-methylcholanthrene oil solution into the femoral muscle. All consequences of adult animal vaccination disappeared in 30-40 days following antigen administration. Quite different consequences were observed after immunization of the newborn mice. The same antigen (Ehrlich cancer cells homogenate) injected to newborn mice on days 1 and 3 after birth in a dose that failed to develop tolerance not only significantly increased the ascytic tumor transplantation threshold (by nearly 200 times for sarcoma 37 cells and by nearly 400 times for Ehrlich cancer cells) in adult animals but also led to almost 50% inhibition of cancerogenesis (induced by injection of 20-methylcholanthrene oil solution in the femoral muscle of mature mouse) after three and even after 12 months following immunization. The MTT-analysis did not reveal any noticeable differences in the number and activity of the cytotoxic lymphocytes in populations of splenocytes obtained from the intact mice (control) and from the adult animals which had been exposed to postnatal immunization (experiment).However, after a new vaccination such differences were found. In the populations of splenocytes obtained from control animals, the cytotoxic activity measured on day 10 after vaccination had increased 2.86-fold mainly at the expense of an increased number of effector cells. In the populations of splenocytes obtained from the experimental group of animals the activation was much greater (25.8-fold), being accomplished not only at the expense of an increased number of the effector cells, as observed in the control group, but also at the expense of their higher activity. The kinetic analysis of a mechanism of effector cells/target cells interaction has led to derive equations for estimation of the limiting rates of such interaction and of the equilibrium constants for interacting cells. Analysis of a generally accepted mechanism of the cytotoxic lymphocytes formation, with an account of the kinetic analysis data, has shown that a major reason of low antitumor resistance of animal organism is the negligible population of resting cells--the precursors of antitumor cytotoxic lymphocytes. Newborn mice vaccination does not produce any increase in the number of resting cells of the necessary type. This circumstance explains both, increase of the ascytic tumor transplantation threshold and increase of the resistance to 20-methylcholanthrene action in adulthood. Adult animal immunization does not possess such action. Analysis of the problem leads to the conclusion that the system of organism's antitumor resistance becomes effective only in those cases when, owing to antigen activation of resting cells, the concentration of cytotoxic lymphocytes rises to such an extent that the rate of tumor cell destruction becomes greater than the rate of target cells reproduction.