In Sweden fish is considered to be an important source of dietary Se. Therefore Se status was assessed in forty-one middle-aged men with widely varying fish consumption. Glutathione peroxidase (EC 1.11.1.9) and selenoprotein P in plasma were measured by radioimmunoassay. Plasma Se among the men increased slightly with increasing consumption of fish, but no such increases in the concentrations of glutathione peroxidase and selenoprotein P in plasma were observed. Moreover, no correlation was found between plasma Se and glutathione peroxidase or selenoprotein P. Instead, glutathione peroxidase was significantly correlated with selenoprotein P (r 0.73, P < 0.001), indicating that both glutathione peroxidase and selenoprotein P were functional indicators of Se status in this group. The proportion of plasma Se located in glutathione peroxidase decreased with increasing plasma Se. The results suggest that the Se consumed from fish had no apparent effect on the amount of Se incorporated into the functional selenoproteins of plasma. It is concluded that in some cases selenoproteins may be better biological markers of Se status than the total concentration of Se.