For a quantitative investigation of eosinophil activation in perennial allergic rhinitis, eosinophil cationic protein (ECP) concentrations were measured by a radioimmunoassay in serum, nasal secretions (ECPWN) and in the supernatant of these nasal secretions (ECPsup) obtained from normal subjects and allergic patients. Levels of ECPWN and ECPsup were higher than that of ECPserum. ECPsup showed a positive correlation with clinical severity, despite the lack of a significant correlation with eosinophilia in nasal smears. ECPWN and ECPserum showed no significant correlation with any of these clinical parameters. There was a weak tendency toward an increase in histamine sensitivity of the nasal mucosa of allergic patients with higher ECPsup although this was not statistically significant. These results suggest accumulation and activation of eosinophils in the allergic nasal mucosa, and also indicate that ECPsup may be a clinical parameter of perennial allergic rhinitis.