The delta 12 desaturase of developing safflower seeds responsible for the conversion of an oleoyl moiety to the linoleoyl moiety of phospholipids was further characterized. The protein concentration of the microsomal preparation, the oleoyl-CoA concentration (the primary substrate), short incubation periods, and the addition of lysophospholipids must be controlled to obtain optimal desaturation. No evidence could be obtained to implicate cytochrome b5 as the intermediate electron carrier. Attempts to solubilize the desaturase with a variety of detergents and chaotropic reagents were not successful. Brief exposure of the microsomal preparation to trypsin resulted in rapid loss of activity. The overall evidence would suggest that the delta 12 desaturase requires a reductant (NADPH), a NADPH:electron carrier reductase, an electron carrier, a specific desaturase, and an acyltransferase with oleoyl-CoA as the substrate to acylate lysophospholipid to the active oleoyl phospholipids (presumably phosphatidylcholine or phosphatidylethanolamine). The complexity of this system suggests that purification of the components and a reassembling of the purified components will be difficult.