Protein blocks consist of multiply aligned sequence segments without gaps that represent the most highly conserved regions of protein families. A database of blocks has been constructed by successive application of the fully automated PROTOMAT system to lists of protein family members obtained from Prosite documentation. Currently, Blocks 8.0 based on protein families documented in Prosite 12 consists of 2884 blocks representing 770 families. Searches of the Blocks Database are carried out using protein or DNA sequence queries, and results are returned with measures of significance for both single and multiple block hits. The databse has also proved useful for derivation of amino acid substitution matrices (the Blosum series) and other sets of parameters. WWW and E-mail servers provide access to the database and associated functions, including a block maker for sequences provided by the user.