This study was carried out in Otago, South Island, where most arable land has a low soil selenium content (less than 0.5 microgram/g) and where selenium (Se) responsive diseases in livestock are common. Se concentration in whole blood, erythrocytes and plasma, and activity of glutathione peroxidase (EC 1.11.1.9) were measured in blood from 104 healthy Otago residents, 80 patients with cancer and 66 noncancer surgical patients. The older residents over 60 years had lower blood Se levels (0.047 +/- 0.010 microgram Se/ml blood) than the young and middle-aged (0.060 +/- 0.012 microgram Se/ml). Blood Se levels of cancer patients were no lower than those of elderly subjects and patients without cancer, and were less than half comparable United States values. Blood Se levels were decreasing in two cancer patients, and the lowest values (less than 0.03 microgram Se/ml blood) were obtained for five cancer patients, and two noncancer patients after a long period of inanition; these were similar to values for patients on parenteral nutrition with negligible intakes. Lower blood Se levels were associated with lower serum albumin and enzyme activities. It is suggested that low Se status of cancer patients was more likely a consequence of their illness than the cause of the cancer.