The measurement of eosinophil derived proteins such as eosinophil cationic protein (ECP) and eosinophil protein X/eosinophil derived neurotoxin (EPX/EDN) in biological fluids may be a useful indicator of eosinophil activity in ongoing inflammatory processes. This study was performed on blood samples and illustrates that serum values of ECP in particular, but also of EPX, are mainly a result of spontaneous release during the processing of blood samples. In the presence of divalent cations (Ca²⁺ and Mg²⁺), the amount of released ECP and EPX is dependent upon the incubation temperature and the time before centrifugation and recovery of serum. Moreover, the utensils used for blood sampling may influence the serum levels of ECP and EPX. Thus, standardized sample processing is of paramount importance if the results are to have optimal diagnostic or clinical value.