1. 1. Avian leucosis group-specific antigen protein has been purified from avian myeloblastosis virus and studied by methods suited for the small amounts available (0.2-0.5 mg). 2. 2. Molecular weight determinations using the high speed equilibrium method demonstrate a weight average molecular weight of approx. 23 000, homogeneity of molecular size, and lack of concentration effect on molecular weight. 3. 3. Amino acid analysis was performed with the use of an automatic amino acid analyzer. The number of dicarboxylic amino acid residues exceeds the number of dibasic amino acid residues. Since the protein is positively charged at pH 8.6 one would expect that some of the former are aminated. Proline is present in high concentration. 4. 4. No N-terminal amino acid could be detected using [14C]fluorodinitrobenzene even in the presence of 6 M guanidine hydrochloride or with special conditions of hydrolysis. It is suggested that the N-terminus is blocked. 5. 5. Carboxypeptidase A releases alanine rapidly in amounts approximately equimolar to the amount of protein taken. Alanine seems a likely candidate for the C-terminal amino acid. 6. 6. About 27 peptides were apparent after trypsin treatment. There are approx. 22 residues of arginine plus lysine in the protein, and one would predict 23 such peptides if the amino acids were arranged in a unique sequence. 7. 7. Trypsin pretreatment of the group-specific antigen prevents the complement fixation reaction, implying that structural integrity of this protein is required for its antigenicity. 8. 8. It is concluded that group-specific antigen is a suitable protein for further investigation because of its interesting properties, and because it may represent a common gene in an important group of RNA oncogenic viruses.