~~ Fraction 1 2 3 4 Mirjana B u d i n c ' e v i C , i'. V r b a S k i , J . T u r k u l o v a n d E t e l k a D i m i C * Institute of Chemistry and Faculty of Technology, University of Novi Sad, YugoslaviaParticles size (mm) 2 0.63 0.40 -0.63 0.25 -0.40 5 0.25Antioxidant activity of ethanolic extracts of the dried unripe fruit of Osage orange (Maclura auranriaca Nutt.) was studied using raw sunflower oil as substrate. Antioxidant activity of both the ethanolic extract and powder of the same plant was also assessed on the basis of measurements on the Rancimat apparatus with lard as the substrate and by determination of the peroxide number. It was established that the antioxidant activity in the case of powder increased with a decrease in the particle size. The antioxidative power of the powder of the Osage orange fruit was compared with that of powders of following plants:Asclepias syriaca L.. Asfrugalus onobrychis L., Thymus marschallianus Willd., and Oenorhera biennis L., using the Rancimat apparatus. Citric acid exhibited a strong synergistic effect in the combination with both the ethanolic extract and powder obtained from the fruit of Maclura auranriaca Nutt.