Sublethal effects caused by exposure to contaminants can reduce fitness in seabirds and ultimately affect their populations. Red blood cells are particularly susceptible to oxidative damage caused by certain pollutants such as oil. This damage can result in injury to hemoglobin, which aggregates and can be observed as inclusion bodies within erythrocytes, called Heinz bodies. The aim of our study was to determine the hematological biomarkers of oxidative damage in seabirds to evaluate its possible effects on the birds' health. Blood samples were taken from nine specimens of seabirds admitted to Aquarium Mar del Plata in Buenos Aires Province, Argentina. Eight Magellanic penguins (Spheniscus magellanicus) and one black-browed albatross (Thalassarche melanophrys) were sampled. Blood was collected in tubes with EDTA for the determination of reticulocytes and in capillary tubes for the determination of hematocrit. Blood was incubated with brilliant cresyl blue and smears were made for the quantification of reticulocytes and Heinz bodies. In both cases, the number of cells/1000 mature erythrocytes was calculated. Hematocrit and reticulocyte values ​​were within the normal range for birds. The presence of Heinz bodies was not observed in any of the birds. Additionally, no anomalies in these biomarkers were observed in two birds with signs of fuel on their feathers, but without signs of oil. This is in agreement with what was found for other bird species, where the values ​​of Heinz bodies in birds from places not impacted by oil were zero or close to this value. Our results constitute the first evaluation of these markers in these two species of seabirds. Although biomarkers related to oxidative damage to red blood cells were not found to be increased, the information presented in our study allows us to establish baselines for the evaluation of future impacts associated with human activity.