The P1 position of protein inhibitors and oligopeptide substrates determines, to a large extent, association energy with many serine proteinases. To test the agreement of phage display selection with the existing thermodynamic data, a small library of all 20 P1 mutants of basic pancreatic trypsin inhibitor (BPTI) was created, fused to protein III, and displayed on the surface of M13 phage. The wild type of displayed inhibitor monovalently and strongly inhibited trypsin with an association constant of Ka = 3 x 10(11) M(-1). The library was applied to select BPTI variants active against five serine proteinases of different specificity (bovine trypsin and chymotrypsin, human leukocyte and porcine pancreatic elastases, human azurocidin). The results of enrichment with four proteinases agreed well with the available thermodynamic data. In the case of azurocidin, the phage display selection allowed determination of the P1 specificity of this protein with the following frequencies for selected P1 variants: 43% Lys, 36% Leu, 7% Met, 7% Thr, 7% Gln.