To elucidate whether or not histamine can be released by antigen from the nasal surface layer in sufficient quantities to produce nasal manifestation of allergy and which among three factors, i.e., the number of surface basophilic cells, the histamine release from these cells, or the mucosal histamine sensitivity, is the most responsible for the degree of nasal allergen provocation, we determined the allergen-induced histamine release from specimens of the mucus-epithelial layer of patients with nasal allergy and also counted the number of basophilic cells in these specimens after staining by Hansel's method. In addition, we evaluated the histamine sensitivity of the mucosa by the end-point test. The net histamine content released from the specimens of the nasal surface ranged from 25.5--310.7 microgram/cm3, which was found to be sufficient to produce nasal manifestation of allergy. Thye correlation coefficients were calculated as the relationships between the degree of provocation reaction and the level of histamine released from the nasal surface, the basophilic cell number, histamine releasability and histamine reactivity. these values were 0.645, 0.440, 0.481, and 0.155, respectively, and, when they were analyzed statistically, the most significant correlations to the degree of provocation reaction were found to be those between the degree of provocation reaction and the histamine content released from the nasal surface, the basophilic cell number, or the histamine releasability and histamine reactivity (p less than 0.01), in that order.