It has been demonstrated that persistent organic pollutants (POPs) can affect the immune system of mammals and birds. In this study, the concentration of different POPs and leukocytes in blood samples from three chinstrap penguin (Pygoscelis antarctica) populations was analysed in order to assess the impact on haematological parameters. Using blood sample smears, basophils, eosinophils, heterophils, lymphocytes and monocytes were quantified. Mature and immature red blood cells were counted and cell alterations in both white and red blood cells were analysed. At the same time, whole blood was analysed for POPs. The results showed that contaminants, such as dichlorodiphenyltrichloroethane and its metabolites (ΣDDT), as well as polychlorinated biphenyls (ΣPCB), had significant correlations to eosinophils, lymphocytes and heterophils. This indicates possible immunohaematological alterations derived from exposure to such contaminants. Cytological alterations were also observed, such as cytotoxic granules, toxic heterophils, and atypical and granulated lymphocytes, which would demonstrate that these seabirds are being exposed to stress agents that could be producing some alterations at a leukocytary cellular level.