The physiological function of the granule contained cytotoxic ribonuclease Eosinophils contain a large amount of ECP but the question is why? What is the function of this protein? There is a constitutive baseline level of the eosinophils in many tissues and certain stimuli cause elevated production and influx of eosinophils in different organs. Moreover levels of the ECP in tissue and peripheral blood robustly correlated with the number of eosinophils present, which might be indicative that the function of ECP is also key to the role of eosinophils (see table 1 ). Since the discovery of ECP in 1977 [8] it has been used and evaluated as a biomarker to assess activity in various inflammatory diseases. This analysis has given indirect information of the proteins role in disease. For a comprehensive review of advantages and pitfalls of the usage of ECP as a biomarker in allergic disease see ref [42] . Furthermore, a number of in vitro studies have addressed the direct functional activities of the protein. Detailed following is a comprehensive review of these studies with summaries in table 1 and 2. To simplify comparison the concentrations used have been recalculated to μg/mL using the mean M w of 19.000 for the native protein (average of 16-22 kDa).