Lymphocytes from 80% of patients with infectious mononucleosis in this study failed to produce macrophage migration-inhibition factor in response to partially purified early antigen of Epstein-Barr virus or to tetanus toxoid, whereas lymphocytes from normal subjects did produce this lymphokine. Subsequent analysis of serum from the patients with infectious mononucleosis revealed a serum factor that completely abrogated antigen-specific inhibition of migration by human leukocytes as well as lymphocyte blastogenesis. The serum blocking factor was present in sera from 11 (73%) of 15 patients with infectious mononucleos but only in sera from two (13%) of 15 normal subjects. Samples of serum from five of the patients with infectious mononucleosis and five normal subjects were fractionated with use of Sephadex G-200 gel filtration, and the eluants were assayed for several substances known to inhibit cell-mediated immunity. Serum blocking factor activity could be demonstrated only in fractionated sera from patients with infectious mononucleosis. The serum blocking factor is postulated to be either a soluble immune complex or some as yet unidentified immunoregulatory globulin contained in the IgG fraction of human serum.