Analogs of the antithrombin peptide hirudin54-65 with C-terminal modifications have been synthesized in order to examine the requirements for alpha-thrombin inhibition. The C-terminal residue, Gln65, could be replaced with L-amino acids or amino alcohols with neutral or charged hydrophilic side chains without greatly affecting the peptide's antithrombin potency as determined by inhibition of thrombin-induced clot formation in human plasma in vitro. Derivatives with D- or L-amino carboxamides at position 65 had significantly reduced potency, but still retained activity. Deletion of residue 65 with conversion of residue 64 to the amide or alcohol derivative resulted in a three-fold loss of potency. In addition to these results the solid-phase synthesis of peptide alcohols via direct displacement of p-nitrobenzhydrylideneisonitroso resin attached peptides with the desired C-terminal amino alcohol is reported.