Thirty-seven patients--aged 2 to 58--with clinical and hematological signs of infectious mononucleosis were studied. Thirty patients gave serologic evidence of Epstein-Barr virus infections; antibodies to viral capsid antigen (VCA) appeared very early (only two seroconversions and 4 cases of increased titer were detected); antibodies to early antigen (EA) occured in 25 patients, but only 9 had high titers; antibodies to nuclear antigen (EBNA) appeared late in the course of the disease. VCA-specific IgM antibodies occurred in 25 cases (they usually disappeared during the 2nd month). The transforming EB virus was found in the saliva of the 2 patients where we looked for it. Heterophile antibody responses occured in only 17 patients; among children about half of them had a positive Paul-Bunnell-Davidsohn test. After the age of 40 (3 cases) heterophile antibodies were no longer found. Three patients had another recent viral infection at the same time as the primary EBV infection (two cases of rubella, one case of adenovirus infection). Of the remaining 7 patients (having either no antibodies to EBV or antibodies of past infection), 2 had cytomegalovirus infections and one rubella.