1. 1. The N-terminal amino acids of the soluble proteins from 10 procaryotes and 5 eucaryotes have been determined by the Sanger procedure using tritiated 1-fluoro-2,4-dinitrobenzene. In addition, the amino acids occupying the second position in the proteins from some of the bacteria have been examined using radioactive Sanger reagent after removal of the N-terminal residues by Edman degradation. 2. 2. Methionine, alanine and serine are the major N-terminal amino acids of the proteins from the procaryotes. The relative amounts of these amino acids depend upon the organism examined, but the order is usually methionine > alanine > serine. The N-terminal amino acids of the soluble proteins from eucaryotes vary more than those of bacterial proteins but consistently high quantities of alanine and serine are found. Relatively large amounts of other amino acids are also found in the N-terminal position of the proteins of individual eucaryotes, especially in the case of the nucleated microorganisms. 3. 3. The amino acids occupying the second position in the proteins from procaryotes are more varied than the N-terminal residues. Only the combination of leucine plus isoleucine and threonine are found in higher amounts than would be predicted from random distribution based on amino acid analyses of the procaryotic proteins. 4. 4. A possible basis for the specificity of removal of methionine from the N-terminals of newly initiated procaryotic proteins is discussed.