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Distinct differences in the intensity and dynamics of the infection with cysts of toxoplasms of law virulent (Czech) strain of brain of mice of three different pure lines and strainless ones differing in the sensitivity to the agent during a chronic period of infection have been noted. In sensitive mice of AKR line the period of formation and growth of cysts as well as the period of their elimination from the brain is longer than in less sensitive ones. In this connection cysts in the brain of these mice turned out to be larger than in the brain of less sensitive mice of CC57BR line. Activation of the infection in the brain, revealed microscopically, is observed most commonly in mice of AKR line (40%), more seldom in CC57BR (9.1%) and strainless ones (7.7%) and is not observed at all in resistent BALB/c. Lethality of these mice was by 9 months of infection 87, 40, 41 and 5.3%, respectively. 1-1.5 year after the infection in animals without relapses the infection level of brain was equally low.

The diameters of toxoplasms' cysts in native preparations of brain of bredless and inbred mice (AKR and CC57BR) infected with the toxoplasms of Czech low-virulent strain were measured. It is shown that the growth of cysts ceases by the 7th week from the infection and the ratio between cysts of different sizes does not change for 38 weeks and more. The curve of the ratio between cysts of different sizes, which in the period of infectious process stabilization is close to the even distribution curve, is apparently a result of primary parasitemia. By the 1st year of the infection a reactivation of the infection in some animals occurs judging by the rise of the number of small cysts. Stabilization and then reactivation of the infection are correlated with the dynamics of humoral antibodies.

The conditions of the increase in the virulence of toxoplasma lines of the Czech low-virulent strain obtained by means of the infection of mice or chicken embryos with one cyst washed three times in physiological solution were investigated. It has been shown that all the 12 were potentially virulent since in the process of frequent (in each 7 days) passage through sensitive animals (bredless mice) they increased their virulence. The lines of toxoplasms passed only one time through chicken embryos increased their virulence quicker (1 to 10 passages) than lines passed only through mice (18 to 22 passages). At the simultaneous passing of toxoplasms of the same line through sensitive animals (bredless mice) and through more resistant ones [mice of lines AKR and BALB/c] toxoplasms increased their virulence during the 4th passage and did not change it up to the 25th passage, respectively.