Experimental studies of infection of Apodemus flavicollis, and for comparison of rickettsiaemia in Clethrionomys glareolus and of susceptibility and antibody formation in white mice, with rickettsiae of the SF group isolated in Slovakia, gave the following results: The species A. flavicollis reacted by the formation of antibodies on subcutaneous administration of rickettsiae, strain B, in an amount of 10(0.5) EID50/0.25 ml, whereas the white mouse only in an amount of 10(2.5). Rickettsiae, strain B, administered in an amount of 10(3.5)EID50/0.25 ml subcutaneously to A. flavicollis were found in smears from the liver and spleen of the inoculated animals up to the 25th day following infection, on detection by the isolation test on chick embryo yolk sacs in the spleen on day 5 and 7, in lymph nodes on day 7 and in the brain on day 15 following infection; on detection by the method of injecting suspension from the organs into the haemocoelom fo ticks regularly in the spleen and liver up to the 10th day, in the brain, kidney and lymph nodes regularly up to the 15th and irregularly in the lungs also up to the 15 day, then regularly in the testes up to the 5th day and in the heart, blood and peritoneum up to the 3rd day after infection. Subcutaneous inoculation of rickettsial suspension in an amount of 10(5)EID50/0.25 ml into A. flavicollis and C. glareolus did not produce rickettsiaemia. However, rickettsiaemia was demonstrated in A. flavicollis on day 3 and 7, and in C. glareolus on day 1, 3, 5 and 7 following the bite by female D. marginatus ticks naturally infected by rickettsiae. A. flavicollis mice responded to oral infection by a suspension of rickettsiae, strain B, by the formation of specific antibodies. Subcutaneous administration of various amounts of rickettsiae, strain D, provoked in A. flavicollis and in white mice the formation of antibodies which in A. flavicollis persisted up to the 44th week following infection and reached the values up to greater than 1:1024 between the 2nd and 6th week, while in the white mice they persisted only up to the 12th week following infection and did not exceed the values of 1:256 in the course of the first 6 weeks. The results suggest that the species A. flavicollis will probably play an important role in the circulation of rickettsiae in nature.