The metabolism of 75Se-selenite and 75Se-selenomethionine (SEM) in chick blood was studied in vitro. 75Se-selenite was rapidly taken up by erythrocytes and then subsequently released into the plasma in a protein bound form. However, 75Se-SEM showed a more gradual and continuous build up in erythrocytes over the 12 hour incubation period, according to a hyperbolic type function. Chromatography of erythrocyte lysates on Sephadex G-200 indicated that 75Se-selenite was incorporated into glutathione peroxidase, whereas 75Se from SEM was mostly incorporated into hemoglobin. The elution pattern (on Sephadex G-200) of plasma 75Se-selenoproteins from 75Se-selenite or 75Se-SEM showed that radioactivity was associated with two peaks corresponding to molecular weight 440,000 and 89,000. Binding of 75Se from either selenite or SEM to plasma proteins was dependent on the presence of erythrocytes. Addition of reduced glutathione and glutathione reductase to plasma produced the same effects as erythrocytes on binding of 75Se from selenite, but not from SEM, to plasma proteins. Dialysis experiments and treatment with beta-mercaptoethanol before addition of trichloroacetic acid suggested that 75Se in these selenium containing proteins is bound as a selenotrisulfide bond. This was confirmed by chromatography of the released radioactivity on an amino acid analyzer. Based on these results, an in vitro model of selenium metabolism in blood is postulated.