Isoxazoles of formula (A) or (B) are inhibitors of HSP90 activity, and useful for treatment of, for example cancers: (A), (B) wherein R.sub.1, is a group of formula (IA): --Ar.sup.1-(Alk.sup.1)p-(Z).sub.r-(Alk.sup.2).sub.s-Q, wherein in any compatible combination Ar.sup.1 is an optionally substituted aryl or heteroaryl radical, Alk.sup.1 and Alk.sup.2 are optionally substituted divalent C.sub.1-C.sub.6 alkylene or C.sub.2-C.sub.6 alkenylene radicals, p, r and s are independently 0 or 1, Z is --O--, --S--, --(C.dbd.O)--, --(C.dbd.S)--, --SO.sub.2--, --C(.dbd.O)O--, --C(.dbd.O)NR.sup.A--, --C(.dbd.S)NR.sup.A--, --SO.sub.2NR.sup.A--, --NR.sup.AC(.dbd.O)--, --NR.sup.ASO.sub.2-- or --NR.sup.A-- wherein R.sup.A is hydrogen or C.sub.1-C.sub.6 alkyl, and Q is hydrogen or an optionally substituted carbocyclic or heterocyclic radical; R.sub.2 is (i) a group of formula (IA) above or (ii) a carboxamide radical; or (iii) a non aromatic carbocyclic or heterocyclic ring wherein a ring carbon is optionally substituted, and/or a ring nitrogen is optionally substituted by a group of formula -(Alk.sup.1)p-(Z).sub.r-(Alk.sup.2).sub.s-Q wherein Q, Alk.sup.1, Alk.sup.2, Z, p, r and s are as defined above in relation to group (IA); and R.sub.3 is hydrogen, optionally substituted cycloalkyl, cycloalkenyl, C.sub.1-C.sub.6 alkyl, C.sub.1-C.sub.6 alkenyl, or C.sub.1-C.sub.6 alkynyl; or a carboxyl, carboxamide, or carboxyl ester group. ##STR00001##