Enduring changes in the surrounding environment cause long-term stress in birds, which can affect the activity of the immune system and lead to changes in heterophil-to-lymphocyte ratios (H:L). The aim of this paper is to investigate if the H:L ratios of male and female Great Tits Parus major during the first breeding attempt differ between two sites of dissimilar habitats – an urban parkland site and a deciduous forest site. In the final model we found a difference between sites, while separate submodels for the study sites showed that females had higher H:L ratio than males in the urban parkland site, with the opposite result being found in the forest site. We suppose that the pattern of variation in H:L ratios we found may result from greater parental expenditures of females as compared to males. Such difference in the H:L ratio between habitats could be the reason for the substantial difference in trophic conditions between the forest and park areas during the breeding of tits.