1. Plasma, platelet and erythrocyte glutathione peroxidase activities and serum lipid concentrations were measured in patients with ischaemic heart disease and matched control subjects. 2. Mean plasma and platelet glutathione peroxidase activities were significantly lower in the patients with ischaemic heart disease. Erythrocyte glutathione peroxidase activities and serum lipid concentrations were similar in patients with ischaemic heart disease and control subjects. 3. No correlations between plasma, platelet and erythrocyte glutathione peroxidase activities were observed. 4. The combination of plasma and platelet glutathione peroxidase activities provided an 86% discrimination between patients with ischaemic heart disease and matched control subjects. 5. Our data suggest that plasma and platelet glutathione peroxidases may be significant risk factors for ischaemic heart disease. Plasma glutathione peroxidase is a previously unrecognized risk factor.