A decapeptide positional-scanning synthetic-peptide combinatorial library (PS-SPCL) made up of four trillion (4 x 1012) decapeptides was synthesized; its use is illustrated here for the study of a peptide-antibody interaction. This library was prepared by a chemical-mixture approach using a specific ratio of amino acids empirically determined to give approximately equimolar incorporation of each amino acid during each coupling step. Despite the immense number of decapeptides making up each peptide mixture [approx. 200 billion (2 x 1011)], specific sequences having nanomolar affinities for a peptide-antibody interaction could be readily identified. Upon screening this decapeptide PS-SPCL in this well characterized system, the known six-residue antigenic-determinant sequence was found, with the most specific residues appearing to 'walk through' the ten positions of the peptide library. More importantly, it appears that antibody recognition in this system is stronger when the antigenic determinant is located at the C-terminus of the decapeptide library. Individual decapeptides corresponding to sequences derived from the most active peptide mixtures at each position were synthesized to confirm the results of the screening, 15 peptides were found to have IC50 values between 0.6 and 9.5 nM. four of which were found to be 5-10 times more active than the known six- and 13-residue control peptides. These results further illustrate the power of the positional-scanning peptide library concept, and extend its practical range to a decamer library composed of four trillion (4 x 1012) decapeptides.