Abstract: AIM: To study the effect of rosmarinic acid (RAD) on free radical production and lysosomal enzyme release of rat peritoneal neutrophils (RPN). METHODS: The production of oxygen free radicals and the release of lysosomal enzymes and the cytosolic Ca²⁺i level of RPN were determined with spectrophotometry and spectrophotofluorometry. RESULTS: RAD decreased superoxide anion production of RPN stimulated by formyl Met Leu Phe(FMLP) and phorbol 1,2 myristate 1,3 acetate (PMA), and inhibited hydrogen peroxide production of RPN stimulated by PMA. Also, RAD inhibited lipid peroxidation of RPN induced by Fe²⁺/cysteine and the release of lysozyme and β-glucuronidase after stimulation with FMLP. RAD decreased the cytosolic Ca²⁺i level of the neutrophils. CONCLUSION: RAD inhibits oxygen free radical production of the activated RPN. The inhibitory effect of RAD on the release of lysosomal enzymes might be mediated by inhibiting the increase of intracellular Ca²⁺in the neutrophils.