Partitioning of glycine, lysine and aspartic acid and their oligopeptides in an aqueous dextran-polyethylene glycol two-phase system containing 0.15 M NaCl in 0.01 sodium phosphate buffer, pH 7.3 and 0.11 M sodium phosphate buffer, pH 7.3 was examined. Relative hydrophobicity of the amino acid residues and peptide bonds was estimated and expressed in equivalent numbers of methylene units. Analysis of a series of reversed di- and tripeptides in terms of relative hydrophobicity showed that the additivity principle does hold for the hydrophobicity of short peptides. The relative hydrophobicity of peptides is affected by the ionic composition of aqueous media as well as by the type of amino acid residues forming peptide bonds in a given peptide sequence.