Analysis of soluble Ehrlich ascites proteins by the Sanger procedure revealed methionine, alanine, valine, and glycine as the major NH2-terminal amino acids. The average monomer weights of these proteins calculated from the yields of NH2-terminal amino acids was 144,000. In contrast, the average monomer weight of Ehrlich ascites soluble proteins calculated from the data obtained after electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate was 32,500. The explanation for the disparity in the estimates of average monomer weight obtained by the procedures appears to be that extensive blocking of alpha-NH2 groups by acetate occurs in these proteins, i.e. of the acetate present in the acidic peptides isolated from proteolytic digests of ascites proteins, 23.2 nmol/mg of protein appears to originate from N-acetyl amino acids. These results suggest that approximately 80% of the soluble proteins from Ehrlich ascites cells contain acetate at their NH2-terminal residues. The extensive N-acetylation of proteins does not appear to be limited to Ehrlich ascites cells and may be characteristic of eukaryotic proteins.