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Genetic peculiarities of mice were studied in regards to their susceptibility to toxoplasms. Differences in susceptibility to little virulent toxoplasms in mice of 9 different lines are shown, which manifest at all stages of the infection process or only at acute or chronic stages. The susceptibility of 6 lines of mice to virulent toxoplasms was found to be equally high.

Experiments on cross infection of peritoneal macrophages of mice with Leishmania of reptiles L. gymnodactyli and free cells of abdominal cavity of caucasian Agama (some part of which is composed by fibroblasts) with Leishmania of mammals L. major and L. donovani have shown the possibility of reproduction of the above species both in reptiles and mammals. The persistence of L. gymnodactyli and L. major in macrophages of mice was traced up to 10 days, the abundance of L. gymnodactyli during the whole period of observations being lower than that of L. major. The abundance of the above Leishmania in these cells happened to be higher than in the cells of reptiles. In the cells of reptiles the infection with these three species of Leishmania was eliminated by 5-6 days. More activity internalization of Leishmania of reptiles into cells of reptiles as compared to Leishmania of mammals was revealed that, apparently, reflects a definite degree of their adaptation to existence in reptiles in vivo.

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.