Lipases from microorganisms (Candida cylindracea, Rhizopus arrhizus and Rhizomucor miehei), animal tissue (porcine pancreas) and a higher plant (rape, Brassica napus) have been evaluated as biocatalysts in the esterifications with n-butanol with regard to their substrate specificity towards fatty acids having a cis-4 unsaturation, e.g. docosahexaenoic (n-3 22:6), cis-6 unsaturation, e.g. petroselinic (n-12-18:1), gamma-linolenic (n-6 18:3) and stearidonic (n-3 18:4), as well as cis-8 unsaturation, e.g. dihomo-gamma-linolenic (n-6 20:3) acid, and fatty acids having unusual structures, e.g. cyclopentenyl (hydnocarpic and chaulmoogric), hydroxy (ricinoleic and 12-hydroxystearic) and epoxy (cis- and trans-9,10-epoxystearic) acids. A common feature of all the lipases tested is their ability to discriminate strongly against unsaturated fatty acids having a cis-4, cis-6 or a cis-8 unsaturation as substrates for esterification, whereas cyclopentenyl, hydroxy and epoxy acids are very well accepted as substrates. Esterification of the fatty acids of Hydnocarpus wightiana seed oil with n-butanol, catalysed by each of the above lipases, revealed that, as compared to the cyclopentenyl fatty acids having saturated alkyl chains, gorlic acid - a cyclopentenyl acid having a cis-6 unsaturation - is also strongly discriminated against as a substrate for esterification.