Neutrophilia following exercise is well described in adults and children. However, the effects of exercise on less concentrated leukocyte populations, such as circulating colony forming unit-granulocyte macrophage (CFU-GM), CFU-granulocyte, erythrocyte, megakaryocyte, macrophage (CFU-GEMM), and the various lymphocyte subsets, is not clear. To obtain this information, members of a high school track team were asked to run flights of stairs at their most rapid pace for 10 min. Prior to and 5 min following the exercise, blood concentrations of various leukocyte populations were measured. As expected, the circulating neutrophil concentration increased following exercise (26% increase). However, the largest postexercise increases were in eosinophils (139% increase) and lymphocytes (67% increase). The increase in blood lymphocytes was primarily due to an increase in total T-cells (52% increase) and T-suppressor cells (49% increase), with no significant elevation in T-helper cell or B-cell populations. The exercise did not induce an increase in the immature/total neutrophil ratio or in the number of circulating CFU-GM or CFU-GEMM.