Twenty-three patients with recent infectious mononucleosis were studied for cell-mediated immunity to Epstein-Barr virus (EBV)-associated antigens. By means of a virion-containing antigen preparation, (P3J), leukocyte migration inhibition was demonstrated in 6 of 13 patients with acute infectious mononucleosis (IM) and in 6 of 10 patients studied during the convalescent period. Inhibition with a soluble antigen (S) preparation was seen in only a few patients at all stages of the illness. Lymphocyte blast transformation on exposure to P3J occurred in 4 of 9 patients with acute IM; no other study group reacted. Control donors lacking anti-EBV antibodies did not demonstrate migration inhibition or blast transformation with either P3J or S. Control donors with evidence of previous EBV infection did demonstrate migration inhibition with P3J (8 of 12) and S (3 of 10). These studies indicated that cell-mediate immunity to EBV-associated antigens could be detected by either leukocyte migration inhibition or lymphocyte blast transformation, but neither assay is diagnostic of infectious mononucleosis.