AbstractThe specific activity of the microsomal Δ12‐desaturase system, which transforms oleic acid into linoleic acid, was about 16 pmol/min/mg protein. However, most of the total activity was nonsedimentable even after a 200000×g centrifugation for 100 min. The study of various physicochemical parameters showed that this enzymatic complex, functioning optimally between pH 7 and 8, had low thermal stability. Ca2+, which may cause an aggregation of the microsomes, and Hg2+ completely inhibited the activity, whereas Mg2+, Mn2+, and Zn2+ were activators. The Δ12‐desaturase system was relatively specific toward oleic acid, though isomers of this fatty acid also had an action, either as substrates or as competitive inhibitors, on the activity of the system. The study of the effect of the exogenous oleoyl‐CoA and elaidoyl‐CoA on the specific activity of the Δ12‐desaturase system showed a preference toward oleoyl‐CoA.