In eosinophil granules pH is maintained at 5.1 by an ATPase [3] where the basic proteins are packed forming crystals [2] . The main content of these granules are four proteins, the major basic protein (MBP) present in their cores, surrounded by a matrix built up of eosinophil peroxidise (EPO), the eosinophil protein X/eosinophil derived neurotoxin (EPX/EDN) and ECP. Vesicotubular structures within the granules direct a differential release of these proteins [4] . The granule proteins were all discovered and characterised about one hundred years after the discovery of the eosinophils [5] [6] [7] [8] . ECP is the best know of the proteins, assessed and used extensively as a marker in asthma and other inflammatory diseases. ECP has been scrutinized in a number of functional studies. The aim of this article is to review some of the findings of ECP quantifications in various diseases and set those in context of the experiments that have functionally analysed the protein. The findings will be used as guidance in a speculation of the biological role of eosinophil.