A radioimmunosorbent assay has been developed which allows the detection in serum of a cationic protein derived from eosinophil granulocytes. In 34 healthy individuals the mean level was 31 microgram/1. with a range of 5-55 microgram/l. The serum concentration of 'eosinophil' cationic protein was correlated (p less than 0.001) to the number of eosinophil granulocytes in peripheral blood. Quantitation of 'eosinophil' cationic protein in serum might be useful in the study of eosinophil granulocyte turnover and function in vivo.