Abstract: The present invention relates to a one-step microbial process for the preparation of 13.beta.-hydroxy- or 14,15-epoxy-milbemycins of the following formulae I and II ##STR1## in which R.sub.1 and R.sub.1 ' represent methyl, ethyl, isopropyl or sec.-butyl or represent the group ##STR2## in which X represents methyl, ethyl or isopropyl, and A and A' represent the group ##STR3## in which R.sub.2 represents hydrogen, --C(O)-CH.sub.2 O-C(O)-CH.sub.2 CH.sub.2 -COOH or --C(O)-CH.sub.2 CH.sub.2 -COOH, or mixtures of compounds of the formulae I and II, which process comprises bringing a milbemycin of the formula III ##STR4## dissolved in a liquid phase, in which R.sub.3 represents R.sub.1 or R.sub.1 ' and R.sub.4 represents A or A', and R.sub.1, R.sub.1 ', A and A' have the meanings given for formulae I and II, into contact with a microorganism that is capable of 13.beta.-hydroxylation or 14,15-epoxidation or of both reactions, or into contact with active constituents thereof.