The apicomplexa parasites of the genus Babesia, the etiologic agents of the disease not only in domestic and wild mammals but also in humans, live and reproduce in erythrocytes of the host. Transmission of Babesia species is by tick (Ixodidae) bite. In natural conditions, trans-stadial and trans-ovarian passage occurs in Ixodes ricinus infected with some species of Babesia, e.g. B. divergens. However, there is apparently trans-stadial passage of B. microti in Ixodes sp. only from larvae to nymphs, but trans-stadial passage from nymphs to adults or trans-ovarian passage has not been reported. The present study was undertaken to compare the parasitemia of B. microti infection in BALB/c and F1 (B10 x CBA) mice by two different methods: intraperitoneal injection of parasites or infection by the oral route. In both groups, experimental mice were inoculated with 5 x 10(7) infected erythrocytes in 100 microliters of blood. Babesia infection was acquired by all mice infected intraperitoneally with maximum 57% of parasitemia on day 6 post infection (pi) in F1 (B10 x CBA) mice and 40% of parasitemia on day 8 and 10 pi in BALB/c mice. Ten of 27 (37%) BALB/c mice infected by oral route showed low parasitemia (9%) during first two weeks pi. In this group of mice the pick of parasitemia (26%) was observed on day 22 pi. In both groups of infected mice the period of prepatency ended between days 35 and 40 pi. Experiments have confirmed that the maintenance of babesiosis may be continued in the absence of a tick vector. Demonstration, under experimental conditions, of infection of Babesia by oral route may suggest that in nature cannibalism of rodents, occurring under certain circumstances, can be considered as a natural way of oral transmission of B. microti.