Chronic fatigue syndrome (CF5) is a special, strictly defined disease which evidently has a clear immunological and chemical basis with a corresponding biologcckl marker: level of ribonuclease L protein fragments in the peripheral blond mononuclear cells as the rado of fragmented ribonuclease L to native ribonuclease L.

        Key words: chronic fatigue syndrome, interferon I, 2' - 5' oligoadenylate synthetase, ribonuclease L