Our comparative analysis of the myelograms of these lake and pond frogs has revealed a higher erythropoietic activity of the bone marrow in the conditionally "background" amphibians (group 1) as compared to the individuals from urbanized (group 2) territories. Restructuring of the bone marrow cellular composition and activation of the myeloid cells were the adaptive reaction of the amphibians to living in polluted conditions of their aquatic environment. The response variability of particular myelopoiesis elements reflected the plasticity of the organism’s compensatory abilities.