Migration of blood monocytes from patients with acute infectious mononucleosis and from normal controls was measured against chemotactic factors in serum. Moncytes from patients with acute infectious mononucleosis showed decreased migration as compared with that of control monocytes. However, serum from patients with infectious mononucleosis contained normal or above normal amounts of chemotaxins for monocytes. The migratory defect of monocytes from patients with infectious mononucleosis was reversible within three months after the onset of diesease. The cause of this monocyte migration defect in infectious mononucleosis is though to be an in vivo blockade of receptors on monocytes for chemotaxins, and it is speculated that this defect can partially explain the explain the ablated delayed-hypersensitivity skin reactions in this disease.