Recently monocyte chemotactic and activating factor (MCAF) has been reported to be a potent histamine releasing factor (HRF) for human basophils. In this study we investigated the localization and biological activity of MCAF in human nasal mucosa. To investigate the localization of MCAF within the nasal mucosa we performed specific immunohistochemical staining of sections of large pieces of the inferior turbinate obtained by conchotomy from patients with nasal allergy and without nasal allergy and compared them. Sections of nasal mucosa from allergic subjects stained with anti-MCAF mAb demonstrated the presence of many MCAF-positive cells in the lamina propria especially around small vessels and excretory gland, but only a few positive cells were found in the nasal mucosa of normal subjects. We cultured allergic nasal mucosa and sinus mucosa from patients with chronic inflammation for 2 days and detected MCAF by western blot analysis of the culture supernatants. One portion separated from sinus mucosa supernatant by column chromatography also released histamine from human basophils. Mean release was 3-4% and the difference from the control was statistically significant (p < 0.05). In the human nasal mucosa it seemed to exist IgE independent histamine releasing system sustained by MCAF producing cells. These results suggested that MCAF is one of the important mediators causing histamine release in the late phase reaction and in protracted inflammation of the nasal mucosa.