Gel filtration of avian tumor virus proteins in 6 m guanidine hydrochloride clearly resolved seven major protein species. The antigenic activity of these proteins was recovered in good yield after removal of the denaturing solvent, permitting a correlation of specific polypeptides with the principal antigens of the virion. Two of the proteins, of molecular weights 70,000 and 32,000, contain carbohydrate and are situated on the viral membrane, as shown by their being accessible in the intact virus to specific antibodies. Four proteins, with molecular weights (in guanidine) of 27,000, 19,000, 15,000, and 12,000, have different group-specific (gs) antigens and are enclosed within the viral membrane. The smallest protein, with a molecular weight of 10,000, has not previously been described; it is not detectable with antisera and possesses a mobility identical to that of one gs protein when subjected to electrophoresis in polyacrylamide gels in the presence of sodium dodecyl sulfate. Of the proteins lacking carbohydrate, three are present in the virion in a molecular ratio of 2:2:1, and the two others, present in almost equal amount, are rich in lysine and arginine.