Observations on 40 known a-amino acetylated polypeptide chains reveal characteristic properties. Types of residues in the acetylated position are limited, with serine and alanine in about three quarters of all cases. The distribution is different (P < 0.001) from that of N-terminal residues in proteins in general. Inside the N-terminal regions the distribution of branched-chain residues in acetylated polypeptides is different (P < 0.01) from that found in proteins in general. Isoleucine is over-represented, long hydrophobic segments are absent and at some positions charged residues are common. These properties axe observed regardless of origin and function of the proteins. The differences between acetylated and nonacetylated proteins in general are also noticed in species variation of acetylation status in a single type of protein. Acetylated polypeptides usually have completely modified N-terminal residues and are frequently structural proteins. Interpretations of these facts suggest that common structural characteristics determine the acetylation of protein ~t-amino groups, and that this modification has an important functional significance. Structural characteristics are concluded to involve the terminal residue as well as the terminal region, which is likely to be exposed and often to have little regular structure. Removal of the charged ~-amino group seems not to be a conformational prerequisite for the protein, but a protective function of acetylation towards proteolysis is possible.