Two peaks of protein-bound 75Se are seen on the gel filtration chromatogram of rat plasma when the rat has been fed a 0.5 ppm selenium diet and no NaCl is added to the Tris-HCl elution buffer. One of those peaks is absent from the plasma of rats fed a very low selenium diet. Poly-acrylamide gel electrophoresis of the 75Se peaks suggests that each may have more than one protein component. Dialysis studies indicate that the 75Se is not in the form of selenotrisulfides.