Epstein-Barr virus (EBV) is present, as both latent and replicating virus, in most healthy individuals, and disturbance of the normal host-virus balance may cause a variety of diseases. A number of diagnostic tools can he used, depending upon the particular condition investigated, however, serology is the best method for diagnosis of primary EBV infections. In approximately 85% of cases of mononucleosis, the primary EBV infection can he diagnosed by an assay for heterophile antibodies. The presence of immunoglobulin G (IgG) and IgM to antigens from the replicative cycle of the virus, in combination with absence of antibodies to the EBV nuclear antigens (EBNA 1-6), is a diagnostic for a primary EBV infection. Serological tests for elevated IgA levels to various EBV antigens have been used to screen for EBV-associated nasopharyngeal carcinoma. In those with AIDS with EBV-associated lymphomas of the brain, EBV genomes can always be detected with polymerase chain reaction on cerebrospinal fluid. Epstein-Barr virus involvement in tumours, other than those of the brain and solid organs, can be verified by in situ demonstration of EBV-DNA or by immunocytochemical techniques for viral antigens.