ALTHOUGH the humoral immune response to Epstein-Barr virus (EBV) in infectious mononucleosis (IM) has been studied extensively1,2, little is known about cell-mediated immunity (CMI) to EBV in the disease. We describe here the status of CMI to EBV, and the presence of a blocking factor which can abrogate leukocyte migration inhibition, in patients with IM.