The isolation and characterization of a new eosinophil-derived protein, eosinophil protein X (EPX) is here described. The purified EPX is a highly basic glycoprotein and shows one band on sodium dodecyl sulphate polyacrylamide gel electrophoresis and on agarose electrophoresis. The molecular weight of the unreduced protein was 23,000 and after reduction 19,000. EPX did not precipitate antibodies prepared to other eosinophil-derived proteins such as eosinophil cationic protein (ECP) or major basic proteins (MBP) or antibodies prepared to a number of neutrophil-derived proteins. The eosinophil origin of EPX was indicated by the linear correlation between the amount of EPX and the percentage of eosinophils in mixed leucocyte extracts prepared from normal individuals. The EPX content was estimated to be on average 10 micrograms/10(6) normal eosinophils. EPX from normal eosinophils and from eosinophils from one patient with hypereosinophilia seemed to be identical.