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Haematological investigation is an important component of stress and immune response assessment and holds great potential for evaluating amphibian welfare. The olm (Proteus anguinus) is a neotenic, cave-dwelling salamander endemic to the Dinaric Karst and classified as a vulnerable species by the IUCN. The aim of the present study was to gain basic knowledge about the haematology of this endangered species and to establish reference values of leucocyte differential counts. Blood smears were obtained from recently captured animals and compared to smears from the same animals in long-term captivity (up to five years). The elongated erythrocytes are nucleated and are among the largest in vertebrates, which is also reflected in their lower concentration per millilitre of blood. The circulating blood can contain mitotically active, immature erythrocytes (erythroblasts), indicating that erythrocytes of the olm can undergo proliferation and maturation in circulating blood. Olms show a typical urodele leucocyte profile, with lymphocytes predominating, followed by neutrophils, although the proportion of lymphocytes is higher than in other urodeles. Eosinophil and monocyte counts were relatively low, while basophils were nearly absent. We found no statistically significant differences in leucocyte counts or in neutrophil to lymphocyte ratio neither between recently captured olms and same animals during captivity nor between the sexes, except for lower neutrophil count in captive females. Thus, no differences in stress levels between recently captured olms and captive olms were detected via haematological investigations. The significance of these results is discussed in the context of detecting stress and immune responses in olms. [ABSTRACT FROM AUTHOR]