An isomer, enantiomer, diastereoisomer or tautomer of a compound, represented by formula I: ##STR00001## wherein: A is O, S, NR.sup.1, or CR.sup.1, wherein R.sup.1 is defined herein; - - - - - represents either a single or a double bond; R.sup.2 is selected from: H, halogen, R.sup.21, OR.sup.21, SR.sup.21, COOR.sup.21, SO.sub.2N(R.sup.22).sub.2, N(R.sup.22).sub.2, CON(R.sup.22).sub.2, NR.sup.22C(O)R.sup.22 or NR.sup.22C(O)NR.sup.22 wherein R.sup.21 and each R.sup.22 is defined herein; B is NR.sup.3 or CR.sup.3, with the proviso that one of A or B is either CR.sup.1 or CR.sup.3, wherein R.sup.3 is defined herein; K is N or CR.sup.4, wherein R.sup.4 is defined herein; L is N or CR.sup.5, wherein R.sup.5 has the same definition as R.sup.4; M is N or CR.sup.7, wherein R.sup.7 has the same definition as R.sup.4; Y.sup.1 is O or S; Z is N(R.sup.6a)R.sup.6 or OR.sup.6, wherein R.sup.6a is H or alkyl or NR.sup.61R.sup.62 wherein R.sup.61 and R.sup.62 are defined herein; and R.sup.6 is H, alkyl, cycloalkyl, alkenyl, Het, alkyl-aryl, alkyl-Het; or R.sup.6 is ##STR00002## wherein R.sup.7and R.sup.8 and Q are as defined herein; Y.sup.2is O or S; R.sup.9 is H, (C.sub.1-6 alkyl), (C.sub.3-7)cycloalkyl or (C.sub.1-6)alkyl-(C.sub.3-7)cycloalkyl, aryl, Het, (C.sub.1-6)alkyl-aryl or (C.sub.1-6)alkyl-Het, all of which optionally substituted with R.sup.90; or R.sup.9 is covalently bonded to either of R.sup.7 or R.sup.8 to form a 5- or 6-membered heterocycle; a salt or a derivative thereof, as an inhibitor of HCV NS5B polymerase.