Abstract: The invention relates to a method for cleaving cervimycin esters. An object of the invention to convert in a simple manner the cervimycin esters which are less active but formed in larger quantity into the highly active cervimycin K that occurs as a minor component in the preparation by fermentation, is achieved by preparing unesterified cervimycins from cervimycin esters with di- or monomethylated malonic acids, by ester cleavage effected by at least one esterolytic enzyme at temperatures of 20° C. to 75° C. and a pH of pH 5.0 to 10.0, preferably pH 6.0 to 9.0.