We examined the effects of the level of internal ⁹⁰Sr and ¹³⁷Cs concentrations on hematological parameters of bats captured in the Chernobyl Exclusion Zone (Ukraine). In total, 68 animals of six bat species were live-monitored with measurement of the whole-body activity concentrations of the two radionuclides (⁹⁰Sr and ¹³⁷Cs) and blood sampling for further analysis. We found statistically significant positive correlations between the blood neutrophil count and whole-body ⁹⁰Sr activity concentration in individuals of Eptesicus serotinus; and between the neutrophil/lymphocyte ratio and ⁹⁰Sr concentration, and between the red blood cell (RBC) count and ¹³⁷Cs concentration in Vespertilio murinus. We did not observe any correlation between concentrations of ⁹⁰Sr and ¹³⁷Cs and hematological parameters in individuals of Pipistrellus nathusii. Moreover, none of these three species showed significant sexual (male versus female) or age (adult versus sexually immature) differences in ⁹⁰Sr and ¹³⁷Cs concentrations. Our results contribute to better knowledge and understanding of the eco-immunology of free-ranging bats in one of the most radioactively contaminated areas in the world — the Chernobyl Exclusion Zone. To our knowledge, this is the first study investigating the relationship between radionuclide contamination and hematological parameters of bats.