Enzyme assays are used to measure the activity or concentration of enzymes in biochemical or cell-based systems. Most enzyme assays are based on the detection of fluorescent, luminescent, or spectrophotometric endpoint signals. In recent years, they have been developed and widely used for diagnostics, mechanisms of action, and inflammatory activities. An enzyme assay essentially works by the conversion of a substrate into a product by the enzyme of interest. In this case, it is extremely important to know the optimal conditions for enzyme activity, as these affect the specificity and efficacy of the assay. For optimal reaction conditions, temperature, pH, and the presence of ions should be considered. In this chapter, the enzymatic assays for the detection of the enzymes N-acetylglucosaminidase (NAG), myeloperoxidase (MPO), and eosinophil peroxidase (EPO) are addressed. These assays are used to assess inflammatory parameters, for example, at the peripheral level in models of viral disease. They are based on an index of neutrophil, macrophage, or eosinophil accumulation in inflammatory tissues from animals by measuring the specific activity of the marker enzymes. The enzyme activity assays discussed here are based on colorimetric reactions compatible with any experimental model in which the respective cells has an active role. The advantage of using these enzymatic assays in inflammation response models is that they are simpler and less expensive compared to techniques such as Western blot or quantitative PCR.