Laboratory animals (mice and guinea pigs) were infected with the isolates of Coxiella burnetii (Derrick, 1939) obtained from bovine milk (M18 and M35) and the ticks Ixodes ricinus (Linnaeus, 1758) and Dermacentor marginatus (Sulzer, 1776) (Kl3 and Kl6, respectively), and with the reference strain Nine Mile. Neither mortality nor lethality occurred with the mice. Antibody response in mice infected with isolates from milk was lower (1 : 16-512) than that from ticks (1 : 32-4096). Onset of seropositivity also occurred later - on the 10th day post-infection (p.i.) for M18 and M35 in comparison with the 7th day for Kl3 and Kl6. In guinea pigs, infection manifested by fever. The fever was less evident in guinea pigs infected with isolates from milk (39.5-40.1°C) than in guinea pigs infected with isolates from ticks (39.5-40.6°C). Partially engorged females of Dermacentor reticulatus (Fabricius, 1794) were inoculated with isolates M18 and Kl3 . No differences in the multiplication of C. burnetii in haemocytes between these two isolates were ascertained.