Epidemiological and scientific research has contributed to the identification of the key role of dietary antioxidants in maintaining human health by preventing oxidative damage to cellular components. The antiradical activity of seven varieties of plums and their protective effects against oxidative stress were comparatively evaluated in mouse granulocytes in this study using a 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) test and a cellular test. The class of phytochemical compounds responsible for the main antioxidant capacities observed was also investigated. The ABTS and cellular tests revealed that the total antioxidant capacities, expressed as vitamin C equivalents, ranged from 691.2 to 2,164.64 mg and from 613.98 to 2,137.59 mg per 100 g of fresh weight, respectively, suggesting that plums are rich in natural antioxidants and appreciably protect granulocytes from oxidative stress. Our results showed a linear correlation between either total phenolic or flavonoid contents and total antioxidant capacity, revealing that these compounds contribute significantly to the antioxidant activity of plums. The major polyphenolics in plums, identified and quantified using reversed-phase high-performance liquid chromatography, were tested to evaluate their protective effect on peripheral blood granulocytes from oxidative stress. The results suggest that individual polyphenolics contribute directly to the total protective effect of plums.