A material of fifteen children aged 10 months to 15 years with infectious mononucleosis (IM) was investigated. The diagnosis was established by demonstration of Epstein-Barr virus (EBV) specific IgM antibodies. The serum concentrations of beta 2-microglobulin (beta 2-m) were significantly increased (p less than 0.01) in the first sample after onset of disease, compared to control groups of children with other infectious diseases and children without diseases. The serum concentration of beta 2-m decreased towards normal range within 3 weeks- 3 months. During the same period, IgM antibodies of EBV decreased to unmeasurable levels, and the IgG titers were almost constant. Serum beta -2m may be a parameter of value in the diagnosis of infectious mononucleosis in childhood, replacing tests for heterophile antibodies (Emni test and Paul-Bunnell test) which are often negative in children suffering from a primary EBV infection, or it may be used for assessment of disease activity.