In this study, the effects of an ethanolic extract from the leaves of Mulberry (Morus nigra) on the PMA-induced respiratory burst in polymorphonuclear neutrophils (PMNs) were investigated. Antioxidant activity and toxicity testing for the leaf extract were also performed. PMNs were purified by density-gradient centrifugation of casein-induced exudates from Swiss albino mice. Antioxidant activity was assessed through DPPH assay and cytotoxicity on PMNs using the MTT reduction assay. Reactive oxygen species (ROS) generation during the respiratory burst of PMNs was evaluated through the luminol-amplified chemiluminescence methodology, with phorbol 12-myristate 13-acetate (PMA, 32 nM) used as stimulus and the effect of the leaves extracts from Mulberry tested at concentrations from 10 to 200 g/mL. The leaf extract showed moderate antioxidant activity (EC50 of 464.6 g/mL). Notably, the amount of total ROS generated during the PMA-induced respiratory burst was significantly (p < 0.05) decreased in PMNs treated with the extracts from M. nigra, relative to PMAinduced respiratory burst in left untreated PMNs. Furthermore, the extract from M. nigra at a concentration of 200 g/mL showed no toxic effects on mice PMNs.