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The erythrocytes could be considered as a valuable model in studying the cytotoxic effect of environmental xenobiotics owing to its particular structure and the important components of their plasma membrane especially lipids and proteins. The aim of our study was to investigate the erythrocyte hemolytic activity of mute swan (Cygnus olor) from Baltic coastal region in two points, i.e. Sopot and Gdynia. These comparisons will show whether mute swans on the Baltic coastal regions have elevated levels of hemolysis plus gender variation in erythrocyte damage. Forty adult mute swans were captured from Sopot and Gdynia in February 2013. Collected birds were measured and weighted, and the blood was sampled. The birds were divided into 4 groups (after the 2nd/3rd year of life): males from Sopot (n = 10), females birds from Sopot (n = 10), males from Gdynia (n = 10), and females from Gdynia (n = 10). Blood samples were collected from the medial metatarsal vein in sodium citrate for erythrocyte sedimentation rate. Blood was stored in tubes and held on the ice until centrifugation. The plasma was removed. The erythrocyte suspension (one volume) was washed with five volumes of saline solution three times. The acid resistance of erythrocytes was induced by 0.1M HCl, while osmotic resistance was induced by incubation erythrocyte suspension with the different solution of urea and NaCl, as well as in solutions of different NaCl concentration (0.1-0.9%). The erythrocyte hemolysis was measured spectrophotometrically. Our study revealed that the highest level of the hemolysis percentage was observed among females from Gdynia, males from Sopot, then females from Sopot, and males from Gdynia. The significant differences between the hemolysis percentage among females from Gdynia and Sopot from 13.5 to 18.0 min after HCl-induced hemolysis was observed. The results also indicated significant differences between the hemolysis percentage among males from Gdynia and Sopot from 14.0 to 19.0 min. Hemolysis percentage showed to be increased not significantly among males from Gdynia with time from hemolysis initiation to 10.0 min but close to linearity with a value of hemolysis percentage among males from Sopot after 25 min of hemolysis initiation. The maximal level of hemolysis percentage was significantly higher among females from Gdynia (at 15.0 min) compared to females from Sopot (at 14.0 min). The maximal level of hemolysis percentage among males from Gdynia and Sopot was noted at 0.5 min after hemolysis initiation. The maximal level of hemolysis percentage was 4.40% and 4.11%, respectively. The highest percentage of hemolyzed erythrocytes after incubation with solutions of various urea concentration was observed both among males and females from Gdynia. The lowest percentage of hemolyzed erythrocytes was exhibited by the erythrocyte suspension of males from Sopot. Comparing the percentage of hemolyzed erythrocytes both among males and females from Sopot, the higher values of hemolyzed erythrocytes was noted among females. The highest percentage of hemolyzed erythrocytes after incubation with solutions of various NaCl concentration was observed among males and females from Sopot and further among males and females from Gdynia after incubation with 0.5-0.9% NaCl solution. Comparing the percentage of hemolyzed erythrocytes after incubition with 0.1-0.4% NaCl solution, the highest percentage of hemolyzed erythrocytes was observed among males and females from Gdynia and then among males and females from Sopot. Our study showed more individual variation, probably related to the impact of the environmental habitat and consumption of ingested prey items by mute swans. Of those males and females, females from Gdynia showed the highest level of hemolyzed erythrocytes.