The nature of 75Se-labeled material in the serum proteins of chicks intubated with 75SeO32− was investigated. Serum was exhaustively dialyzed against 0.9% NaCl and then subjected to nonproteolytic treatments for the release of 75Se activity. Reduction of serum proteins obtained 4 h after 75Se administration with 0.25 M 2-mercaptoethanol, 0.10 ML-cysteine, 0.10 M reduced glutathione, or 0.02 M dithiothreitol in the presence of 8 M urea released 76–89% of the radioactivity within 2 h. Sulfitolysis and dialysis against dilute NaOH, pH 11.5, for 2 days liberated 83% and 90%, respectively. At later times after 75SeO32− administration, progressively less 75Se activity was removed by reduction or sulfitolysis, whereas the alkali treatment remained equally effective. On paper chromatography, the alkali-released material separated into two radioactive areas, one attributable to 75SeO32− and the other tentatively identified as colloidal, elemental selenium. No radioactive selenocystine, selenomethionine, or other ninhydrin-positive material was released. None of the treatments employed to discharge the element caused the release of 75SeO32− from 75Se-labeled selenomethionine or selenocystine standards. The evidence suggests that the bulk of 75Se existed in the serum proteins covalently bound between the sulfurs of half-cystine residues.