We investigated seasonal variation in haematological indices and immune function in the non-migratory Great Tit Parus major over a complete annual cycle. The haematocrit value showed a marked reduction in spring and summer, reaching a lowest value during moult, after which it increased to reach a maximum in winter and spring. The peak in the heterophil to lymphocyte ratio (H/L) during July indicated that Great Tits were the most stressed during the first half of the moulting period. The increase in heterophils and H/L ratio, concurrent with a reduced number of lymphocytes during the breeding season, probably reflected the cost of reproduction in terms of physiological stress and immune suppression. After breeding the number of heterophils and the H/L ratio decreased, reaching a lowest value during winter. The concentration of immunoglobulins followed the seasonal pattern in the number of heterophils, though highest values occurred somewhat later, in July–September during the second part of the moulting period, Our observations indicated large differences in activity throughout the year of different components of the immune system, This suggests differences in function among the components and possibly differences in susceptibility to stress, parasitism and hormones during the annual cycle. When juveniles became independent of their parents, the immunoglobulin concentration increased, whereas other immune measures did not show a significant change. This indicates a rapid increase of at least one component of the immune system after the young fledge.