We investigated possible immunological changes in 15 professional football players before, during and after the sports season. We studied the leucocyte count as well as different functions such as T-lymphocyte proliferation, NK activity, chemotaxis and phagocytosis of neutrophils. Training and competitions did not produce any change in the total number of leucocytes but increased neutrophil counts and decreased T4 lymphocyte counts. We also observed a slight decrease of T-lymphocyte proliferation and a significant decrease of neutrophil functions. On the other hand, training and competitions did not induce significant changes in the number of NK cells nor in the total NK cytotoxic activity. The different change observed tended to normalize after the sports season. Our results suggest a predominant neutrophil function depression in football players during a training season which could partly explain the susceptibility of elite athletes to infections.