Peroxidase activity as a measure of neutrophil populations in inflammatory peritoneal exudates of rainbow trout, Oncorhynchus mykiss (Walbaum)

A quantitative peroxidase assay using tetramethylbenzidine was applied to exudates from peritoneal cavities of rainbow trout which had been inflamed by intraperitoneal injection of Yersinia ruckeri. Most of the peroxidase activity was cell‐associated, with values for extracellular peroxidase below 5% of total peroxidase. No free peroxidase activity was detected in resting cavities. The kinetics of neutrophil numbers and peroxidase activity in peritoneal exudates during the 6 days of inflammation studied followed identical patterns, with a correlation coefficient of 0.996. This shows that the peroxidase assay can be used as an indirect, quantitative evaluation of neutrophil responses during peritoneal inflammation.