The activity of Δ9 desaturase was determined in the microsomal fraction of normal mouse liver and hepatoma SS1K in the presence of the 105,000 × g supernatant. Neither hepatic nor hepatoma soluble fractions were able to modify the low desaturating capacity. Two enzymes from the microsomal electron transport chain associated with Δ9 desaturase, namely NADH-cytochrom b5 reductase and NADH-cytochrome C reductase were also measured. The results indicate that the low Δ9 desaturase activity in hepatoma SS1K could be related to the reduced amount of desaturase.Facultad de Ciencias Médicas