1. Erythrocyte, plasma and whole blood selenium concentrations and glutathione peroxidase (EC 1.11.1.9; GSHPx) activities were measured (1) in 104 healthy New Zealand residents living in Otago, a low-soil-Se area (2) in sixty-four surgical patients, including nineteen patients on total parenteral nutrition and twenty-three cancer patients (3) in fifty-two 'overseas subjects' (twenty-five visitors to Otago from outside New Zealand and twenty-seven Otago residents on return from overseas travel). 2. Blood Se concentrations reflected dietary Se intake; means for Otago patients, healthy subjects and overseas subjects were different 0.043, 0.059, 0.136 micrograms Se/ml blood respectively) and mean for overseas residents was greater than for New Zealand overseas travellers. 3. Erythrocyte Se concentration was always greater than plasma Se, and plasma Se was a smaller proportion of erythrocyte Se for patients compared with the controls. 4. GSHPx activities were different in the three groups, and varied directly with erythrocyte Se until a plateau was reached at approximately 0.14 micrograms Se/ml erythrocytes. 5. Overseas subjects showed no relationship between erythrocyte Se and GSHPx activity. This agrees with some overseas studies and the significance of this finding is discussed. 6. Plasma Se concentration remained the most sensitivie index of short-term changes in Se status, and erythrocyte Se and GSHPx activities for long-term changes in New Zealand subjects. Use of these measurements for overseas subjects with higher blood levels is discussed.