Strenuous, anaerobic exercise leads to an increase of leukocytes that are mobilized from the marginal pool. We have analyzed in human peripheral blood the effect of exercise on the number of CD14(+)CD16(+) monocytes as determined by two-color immunofluorescence and flow cytometry. We show herein that this type of monocyte responds with a dramatic up to 4.8-fold increase. Mobilization does not occur after 1 min at 100 or 200 W but 1 min at 400 W leads to a twofold increase of the CD14(+)CD16(+) monocytes immediately after exercise. The numbers remain high at 5 min and gradually decrease to reach the initial level at 20 min postexercise. After 20 min of rest, the CD14(+)CD16(+) monocytes can be mobilized again by a second exercise. The CD14(+)CD16(+) monocytes appear to be mobilized from the marginal pool where they preferentially home because of a higher expression of adhesion molecules like CD11d and very late antigen-4. Exercise goes along with an increase of catecholamines, and mobilization of the CD14(+)CD16(+) monocytes can be substantially reduced by treatment of donors with the beta-adrenergic receptor blocker propranolol. Mobilization of CD14(+)CD16(+) monocytes by a catecholamine-dependent mechanism may contribute to the increase of these cells in various clinical conditions.