Abstract: A novel isocarbacyclin which is a compound represented by the following formula ##STR1## wherein R.sup.1 represents a hydrogen atom, or a C.sub.1 -C.sub.4 alkyl or alkenyl group; R.sup.2 and R.sup.3 are identical or different and each represents a hydrogen atom, a tri(C.sub.1 -C.sub.7)hydrocarbon-silyl group or a group forming an acetal linkage together with the oxygen atom of the hydroxyl group; R.sup.4 represents a hydrogen atom, a methyl group or a vinyl group; R.sup.5 represents an unsubstituted linear or branched C.sub.3 -C.sub.8 alkyl group which may be interrupted by an oxygen atom, a substituted linear or branched C.sub.1 -C.sub.5 group in which the substituent is a C.sub.1 -C.sub.6 alkoxy group or a phenyl, phenoxy or C.sub.3 -C.sub.10 cycloalkyl group which may be substituted further, a phenyl group which may be substituted, a phenoxy group which may be substituted, or a C.sub.3 -C.sub.