Infectious mononucleosis (IM) and cytomegalovirus (CMV) mononucleosis are caused by a primary infection with related viruses, Epstein-Barr virus (EBV) and CMV. Despite the similarity of clinical manifestations, basic differences exist: (1) The heterophil antibody (HA) response is absent in CMV mononucleosis, whereas it is present in IM. (2) In IM atypical lymphocytosis reflects proliferation of B cells early and of T cells later in the disease course; in CMV mononucleosis the situation appears complex. (3) In blood, EBV is restricted to B lymphocytes, whereas CMV is found in polymorphonuclear and mononuclear leukocytes. (4) Complications of CMV mononucleosis such as hepatitis and pneumonitis may be due to virus cytopathic effect in target organs. Prominent tonsillopharyngitis with adenopathy, and visceral complications of IM are related to lymphoproliferation which is self-limited except in males with a rare familial defect in defense against EBV. Immune complex-mediated pathology may occur in both diseases. (5) CMV is frequently transmitted to a fetus in utero or to an infant during or after birth, and this occasionally leads to severe cytomegalic inclusion disease; vertical transmission of EBV appears to be exceptional. (6) Secondary EBV infections are associated with certain malignancies whereas such an association has not been recognized in the case of CMV. Toxoplasma gondii is another cause of HA-negative mononucleosis. Its complications in the heart, in skeletal muscle and in the central nervous system are related to direct invasion by the parasite. Cellular immunity plays an important role in defense against all three agents.