The invention relates to biologically active molecules, which interact with and inhibit thrombin. The invention particularly relates to molecules of general formula (I): Y.sup.1--(NH--X.sup.1--CO)--(NH--X.sup.2--C.dbd.O)--(NH--X.sup.3--C.dbd.O- )--(NH--X.sup.4--C.dbd.O)--(NH--X.sup.6--C.dbd.O) (NH--X.sup.6--C.dbd.O)--Y.sup.2, wherein the compound can be used for thrombin inhibition, inhibition of fibrin formation, and/or for the inhibition of the formation of a clot. Compounds disclosed herein can also be used for treatingm, diagnosing, and prophylaxis of thrombotic disorders.