Patients with Epstein-Barr virus (EBV) infectious mononucleosis (IM) usually develop heterophilic antibodies and some autoantibodies. Antibodies to rubella, measles, adeno-, entero-, herpes simplex, cytomegalo- and varicella-zoster viruses were titrated in sera from IM patients and matched healthy controls using the complement fixation test (CFT) and the haemagglutination inhibition test. Except for herpes simplex virus and cytomegalovirus, the IM sera had significantly higher arithmetical and geometrical mean antibody titres and showed in most cases higher antibody prevalences in the CFT. The titre rise was most pronounced for rubella and measles antibodies, between 2- and 3-fold. There were no cases of very high titres occasionally seen in IM. The IM sera had higher total IgG serum levels than the controls, 17.27 g/l and 11.8 g/l, respectively (P < 0.001). The present data show that in addition to previously reported high levels of some autoantibodies and of heterophilic antibodies, there is a more general increase in IgG antibodies to commonly occurring viruses. This increase is most likely due to the polyclonal activation of B-lymphocytes following the binding of EBV to the complement receptor CR2 (CD21). When due consideration is given to the possible occasional occurrence of a false positive rubella IgM test, the raised antibody-titres will most likely not interfere with routine diagnostics.