Recently four tissue toxic proteins namely major basic protein (MBP), eosinophil peroxidase (EPO), eosinophil-derived neurotoxin (EDN), and eosinophil cationic protein (ECP) were found in eosinophilic leucocytes. Although the characteristics of these proteins concerning tissue damage in the local site of type I allergic reaction have been investigated mainly in lower respiratory tract, the actual clinico-pathological roles of these proteins in nasal allergy are not clarified. Contrary, eosinophils also have histaminase, arylsulfatase, phospholipase D, which are considered to act on a negative feedback mechanism in allergic reaction through inactivation of chemical mediators. Therefore, estimation of ECP and simultaneously arylsulfatase B in nasal secretion and the sera from patients with nasal allergy may clarify the dynamics of clinico-pathological state, especially in the late phase of allergic reaction in each patients. ECP concentrations in the nasal secretions from 22 patients and in the sera from 12 patients with nasal allergy were measured by RIA method. The activities of arylsulfatase B in the nasal secretions and the sera were also estimated in the same specimens as ECP by measuring its hydrolytic activity using p-nitro cathecol sulfate as a substrate. The results obtained were as follows; 1) There was a significant correlation between ECP concentrations in the nasal secretions and the severities of clinical symptoms, especially the degree of nasal obstruction. ECP concentrations also significantly correlated to the score of eosinophilic leucocytes in the nasal smears. 2) The serum ECP concentrations significantly correlated to the number of eosinophilic leucocytes in the peripheral blood, and also showed slight tendency of correlation to the severity of clinical symptoms.(ABSTRACT TRUNCATED AT 250 WORDS)