Pronounced leukopenia during hibernation was found in bats (Chiroptera) of Karelia - brown long-eared bat (Plecotus auritus), Brandt's bat (Myotis brandtii) and northern bat (Eptesicus nilssoni). A higher content of lymphocytes, eosinophils and basophils was revealed in the northern bat featuring an increased resistance to cold as compared to the Brandt's bat and the brown long-eared bat. The differences in ratio of particular lymphocyte types may be related to biological, ecological and physiological peculiarities of the species studied.