The aim of this study is to investigate how does an acute 4-week submaximal exercise affect both leukocyte and leukocyte subgroups. The study registered 8 healthy male subjects. The subjects were exercised on Monark 814-E and 818 model ergometric bicycles 3 times a week for 4 weeks at above threshold heart rate and 75% of Max VO2. Blood samples were collected three times from the subjects, before starting the exercise, immediately after finishing the last exercise and 2 hours after the last exercise. The samples were analyzed in HemaCell Counter (Skits) apparatus to quantify leukocyte, neutrophil, monocyte, eosinophil and basophil levels. Leukocyte counts obtained in both post-exercise measurements were significantly higher than those before the exercise (p< 0.05). Neutrophil percentages two hours after exercise were higher than those before and immediately after exercise (p< 0.05). Lymphocyte percentages obtained just after exercise were elevated in comparison to those both before and two hours after exercise (p< 0.05). However, lymphocyte percentages two hours after exercise were significantly lower than the pre- and post-exercise percentages (p< 0.05). Percentages of monocytes, eosinophils and basophils in both post-exercise measurements were lower relative to pre-exercise values (p<0.05). In conclusion both acute and 4-week submaximal exercise have significant effects on leukocyte and leukocyte subgroups. © 2010 - IOS Press and the authors. All rights reserved.