Serum levels of interferon gamma, interleukin-6 and neopterin were determined in 15 patients with different forms of Epstein-Barr virus-associated diseases: acute self-limiting infectious mononucleosis, chronic active infectious mononucleosis and X-linked lymphoproliferative syndrome. In patients with acute type of infection, neopterin, interferon gamma and interleukin-6 were elevated in nearly all patients. In contrast, the situation was less clear-cut in the other EBV-associated diseases; particularly interleukin-6 was undetectable in most cases. The results suggest that concomitant measurement of these diverse immune activation markers may provide interesting insights into the interactions between the virus and the host, and may also lead to therapeutic consequences.