Neutrophils are an important component of host defence against many bacterial, viral and fungal infections, and the evaluation of neutrophil function is valuable for assessment of health status [1]. Some literature is non-specific or contradictory about the classification of leucocyte morphology in fish, since morphology alone can vary by species [2e4]. Most studies document that fish neutrophils have very similar morphological and histochemical staining properties to mammalian neutrophils, and can be distinguished by the presence of myeloperoxidase in cytoplasmic granules [5]. Neutrophils are found in fish kidney, spleen and blood, and are commonly increased in inflammatory lesions [6]. There is evidence for phagocytic, chemotactic and bactericidal functions in fish neutrophils and an intense respiratory burst [7e9]. Techniques for obtaining relatively pure neutrophil populations for functional studies have not been previously reported for the fathead minnow (Pimephales promelas Rafinesque, 1820), and the characterisation of leucocyte morphology and cytochemical staining has not been adequately defined.