The present study demonstrates a significant decrease in glutathione peroxidase activity in erythrocytes of 24 patients with multiple sclerosis (MS) when the data are expressed as enzymic units per mg hemoglobin and compared to data from normal controls without known family history of demyelinating diseases. Since selenium is an essential part of glutathione peroxidase, this study also compares the topographic differences in selenium availability (expressed as selenium content of forage) with the prevalence and death rates of MS in the USA. The comparison cannot exclude the possibility of a relationship between low selenium content and high prevalence of MS. The data are discussed in relationship to current theories on the pathogenesis of MS.