SUMMARYProtein IX from adenovirus type 2 was purified by two methods, one from groups of nine hexons obtained by disrupting purified virus by heating in the presence of deoxycholate, and the other by a previously published method. The purified protein was used to obtain a monospecific antiserum. Protein IX was found to possess both sub-group-and type-specific antigenic determinants which were apparently accessible within the groups of nine hexons. Approximately 15 molecules of IX were found per group of nine hexons and from considerations of symmetry it seemed possible that IX was located at the 'corner to edge' contacts between hexons in the icosahedron.The protein in infected cells was found to possess approximately neutral charge as determined by immunoelectrophoresis. This was consistent with the amino acid composition, which showed it to be rich in serine, alanine and leucine with approximately half of its glutamic and aspartic acid residues amidified, and the isoelectric point of 6.0, as determined by two dimensional gel analysis. No free N-terminal amino acid was detectable. It is suggested that a unique tryptophan residue is located at around position 70 from the blocked N-terminus, on the basis of chemical cleavage by BNPS-skatole. Based on one tryptophan residue a total of Io 7 amino acids and a mol. wt. of I I 2oo was deduced. Analysis of sSS-methionine-labelled infected cell extracts in a two-dimensional gel system showed that the synthesis of polypeptide IX could be detected early in infection, i.e. in the presence of an inhibitor of DNA synthesis.

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