Human eosinophils have been shown to contain as major constituents arginine-rich eosinophil cationic protein (ECP). The major component of ECP has a molecular weight of 21,000 and the isoelectric point is above pH 11. The specific biological function of ECP is unknown. Findings of cytotoxic effects on parasites and tissue damage when injected into the central nervous system point, however, to a “destructive” action of ECP. Therefore, it may also be involved in undesirable tissue damage associated with a high eosinophilia. As ECP also activated the Hageman factor and preactivated plasminogen it may provide mechanisms to explain effects of eosinophils on coagulation. Density gradient separation of blood eosinophils has revealed the presence in certain states of high eosinophilia of cell populations which may represent “activated” cytotoxic eosinophils. These were characterized by low density, low content of ECP, increased spontaneous oxygen consumption and increased surface receptors for immunoglobulin and complement. Measurements by a radioimmunoassay of ECP in serum have provided evidence for release of ECP in vivo and have supplied a tool for studies of the involvement of the eosinophil in allergic and other inflammatory reactions.