Chemicals have not conclusively been shown to potentiate human viral infections or Reye's Syndrome, but they have been proven to potentiate viral disease in insects, mice, rats, ducks, and tissue in culture. The data suggest that we cannot rule out virus-chemical interactions in the pathogenesis of human disease. Reye's Syndrome (RS), an encephalopathy with fatty changes of the viscera, is a disease of children with a high mortality and unknown etiology. The most frequently suggested causes of RS are viruses or environmental toxins. It has also been suggested that both may cause RS acting either alone or in concert. Varicella or influenza B are the viruses most frequently associated with RS. Aflatoxin, a fungal toxin, has been the toxin most frequently implicated in RS or a disease that is almost identical to it. The hepatic tissue of 9 children with Reye's Syndrome was examined and was found to contain no aflatoxin. The sensitivity of our test was 5.2 μg of aflatoxin B 1/kg wet hepatic tissue. With some reservation, it is felt unlikely that aflatoxicosis is important in the pathogenesis of Reye's Syndrome in the United States. We also examined the serum of 12 patients with RS for pseudocholinesterase activity. Although the mean value of this activity for the RS patients was 12% less than that for the control group, there was no significant differences (P > 0.05) and, therefore, our RS patients did not demonstrate any effects of pseudocholinesterase inhibitors. It is as yet difficult to demonstrate, except coincidentally, environmental toxins in the pathogenesis of Reye's Syndrome.