Compounds of formula (I): ##STR1## (wherein R represents a group of formula ##STR2## and the corresponding ring-closed lactones, salts (especially alkali metal salts) and esters (especially C.sub.1 -C.sub.5 alkyl esters) thereof may be prepared by subjecting ML-236B, or ML-236B carboxylic acid or a salt or ester thereof to enzymatic hydroxylation, which may be effected by means of microorganisms of the genera Mucor, Rhizopus, Zygorynchus, Circinella, Actinomucor, Gongronella, Phycomyces, Martierella, Pycnoporus, Rhizoctonia, Absidia, Cunninghamella, Syncephalastrum and Streptomyces, or cell-free, enzyme-containing extracts from said microorganisms. The compounds are capable of inhibiting biosynthesis of cholesterol and are thus useful in the treatment of hypercholesteraemia.