A radioimmunoassay was developed allowing measurement of the cytotoxic cationic ECP. The assay, which has a total incubation time of 3.5 hr, is a double antibody assay with radiolabelled ECP, covering the concentration range of 2-200 micrograms/l. Performance data show a detection limit of less than 2 micrograms/l and a cross-reactivity with eosinophil protein X (EPX/EDN) of less than 0.06%. The coefficient of variation (%) within the measuring range was, within assay 4.8-10.4, and total 6.6-12.0. The assay is useful for measurement in various body fluids including serum, nasal secretions and bronchoalveolar lavage fluid, and dilution of samples prior to analysis was generally not required. Sera from 100 apparently healthy individuals revealed a geometric mean of 6.0 micrograms ECP/l and a range (95%) of 2.3-15.9 micrograms/l. The elimination rate of ECP, t1/2, in vivo was estimated to be 65 min when ECP was measured in serum. Comparisons between this assay and a method previously described showed that the new method is superior with regard to precision and assay procedure.