Overlapping symptomatologies between chronic fatigue syndrome (CFS) and chemical sensitivity have been observed by different investigators. Interferon-induced proteins 2-5A synthetase and protein kinase RNA (PKR) have been implicated in the viral induction of CFS. The objective of this study was to measure 2-5A and PKR activity in patients with CFS and toxic chemical exposure. Based on the CDC definition and criteria, twenty CFS patients who were positive for viral genome(s) (mainly HHV6; HTLV II, EBV, and CMV) and did not have any history of exposure to toxic chemicals were included in this study. As a comparison, the second group of patients consisted of twenty individuals from the same geographical area who were negative for viral genomes but had been exposed to methyl tertiary-butyl ether concentration of up to 70 ppb and benzene concentration up to 14 ppb. All patients complained of fatigue and other symptoms overlapping between the two groups. From all 40 patients, blood was drawn, leukocyte extract was prepared and assayed for 2-5A synthetase and PKR activity. Clinical specimens which were positive for viral genomes showed from 2.2-38.7 fold increase in 2-5A activity and 1.3-13.5 fold increase in PKR activities over the background of the healthy controls. Similarly, the second group (negative for viral genomes, but exposed to chemicals) showed a 1.1-29.2 fold increase for 2-5A synthetase and a 1.3-11.6 fold increase for PKR when they were compared to healthy subjects. To elucidate mechanisms involved in viral versus chemical induction of 2-5A synthetase and PKR, MDBK cell lines were cultured either in the presence or absence of HHV6, MTBE, or benzene. 2-5A and PKR activities were measured in all the above conditions. A clear induction of 2-5A and PKR was observed when MDBK cells were exposed to HHV6, MTBE, and benzene indicating that induction of interferon-in-duced proteins are not unique to viruses. We conclude that 2-5A and PKR are not only biomarkers for viral induction of CFS, but biomark-ers to other stressors that include MTBE and benzene.