A direct, rapid, quantitative colorimetric assay to determine neutrophil primary granule degranulation was adapted for use with fathead minnow kidney neutrophils. The assay measures the exocytosis of myeloperoxidase (MPO) using 3,30,5,50-tetramethylbenzidine as a substrate. The assay was validated by comparing the total myeloperoxidase content of neutrophil populations obtained from adult cattle, as a known positive, and fish; evaluating the effects of calcium ionophore (CaI), phorbol myristate acetate (PMA), aqueous solution of b-glucan (MGAQ) and zymosan (Z) with and without cytochalasin B (cyto B) as stimulants of degranulation; determining the kinetics of primary granule exocytosis and detecting changes in degranulation when fish were exposed to stress and anaesthesia with MS-222. The MPO assay detected MPO activity in fathead minnow neutrophils that correlated to neutrophil numbers, confirmed that degranulation was increased when CaI was used compared to other stimulants, determined degranulation peak at 60 min and confirmed decreased degranulation after exposure to handling and crowding stress, with and without MS-222. Therefore, the MPO assay is capable of detecting important differences that may occur in degranulation of fathead minnow kidney neutrophil primary granules and in total neutrophil myeloperoxidase content.