A method and software tool to develop patterns of protein families has been designed. These patterns are intended for the identification of local similarities in arbitrary amino acid sequences with proteins of the SWISS-PROT bank. The method is based on the physical, chemical and structural properties of amino acids. It assembles a 'best set' of elements (a pattern) for a given group of aligned related proteins. These elements provide discrimination between proteins of a family and representatives of other families or random sequences. The method combines the advantages of BLOCKS (automatic generation of multiple elements for protein groups), PROSITE (simplicity of element presentation) and matrices/profiles (different distinctions between amino acids for different positions of aligned sequences). Using our method, a data bank of protein family patterns, PROF_PAT, is produced. This data bank is based on the 27,752 amino acid sequences of SWISS-PROT bank release 24. The characteristics of patterns of 743 related protein groups are described. The results of comparisons of PROF_PAT patterns with the proteins of the SWISS-PROT bank are discussed.