Compounds of formula (I): ##STR00001## wherein R.sup.1 is hydroxyl or sulfonamide derivative; R.sup.2 is t-butyl or --CH.sub.2--C(CH.sub.3).sub.3 or --CH.sub.2-cyclopentyl; R.sup.3 is t-butyl or cyclohexyl and R.sup.4 is cyclobutyl, cyclopentyl or cyclohexyl; or a pharmaceutically acceptable salt thereof, are described as useful as inhibitor of the HCV NS3 protease.