Unsatd. omega-3 fatty acids were concd. in a fish oil (Atlantic mackerel fillets) from 35.5% to 90,4% by complexing satd. and monounsatd. fatty acids with urea in aq. EtOH, removal of the complexes and extn. with n-hexane, stabilized by addn. of alpha-tocopherol (0.2%) and tested for oxidative stability at 2-4 degrees C for 12 weeks. The tocopherol-contg. concs. were stable for 4 weeks. The conc. was recommended as a diet supplement.