The technique of Fmoc chemistry has been applied successfully to the synthesis of branched peptides. The immunogenicity of branched peptides has been compared quantitatively with those of protein-conjugated and resin-linked peptides. Six different peptide sequences were used to immunise rabbits and both antipeptide and anti-protein titres were determined for each serum. The data show that the titres of sera from rabbits immunised with branched peptides were higher than those of rabbits immunised with protein-conjugated peptides which in turn were higher than those immunised with resin-linked peptides. The effect was demonstrated with two strains of rabbits.