An immunologic investigation was undertaken in a patient with atopic dermatitis and infectious mononucleosis complicated by eczema herpeticum. Humoral immunity was normal. The cell-mediated immune (CMI) response was temporarily depressed during the acute phase of the illness as measured by in vivo skin tests, and in vitro tests using T-lymphocyte population, macrophage inhibition factor, and lymphocyte transformation. It is postulated that the depression of CMI caused by infectious mononucleosis precipitated the development of eczema herpeticum in this patient.