Thrombin inhibitors represented by the formula ##STR1## as provided wherein A is e.g. phenylglycyl, and phenylalanyl, .alpha.-methylphenylalanine and .alpha.-methylphenylglycine wherein the amino group is preferably substituted with lower alkyl alkanoyl or lower alkoxycarbonyl, or a bicyclo group e.g. 1,2,3,4-tetrahydroisoquinolin-1-yl. Also provided are a method for inhibiting clot formation in man and animals, pharmaceutical formulations useful in the method and intermediates for the inhibitors.