OBJECTIVE To demonstrate the existence and localization of monocyte chemotactic and activating factor or monocyte chemoattractant protein-1 (MCAF/MCP-1) in human nasal mucosa and to verify its activity as a histamine-releasing factor. METHODS Detection of MCAF/MCP-1 in culture supernatants of nasal mucosa using Western blot analysis and assay of histamine release from basophils induced by these culture supernatants. Detection of MCAF/MCP-1 expression in nasal mucosa of patients with perennial allergic rhinitis using immunohistochemistry. METHODS Twenty-one patients with house dust mite allergy, 7 nonallergic patients, and 5 patients with chronic inflammatory sinusitis participated in the study. All the allergic patients had positive test results for mite nasal allergy, detected by a clinical history, a nasal provocation test, and determination of specific mite IgE antibodies by a radioallergosorbent test. RESULTS In Western blot analysis of supernatants of explant culture of human nasal mucosa, the band corresponding to approximately 13 to 15 kd was observed. This band was considered to be MCAF/MCP-1. These supernatants induced histamine release from basophils (approximately 3%-5% in net histamine release), and anti-MCAF/MCP-1 antibody inhibited this histamine-releasing activity. Immunoreactivity of MCAF/MCP-1 was observed in the nasal submucosa but not in the epithelium. Immunoreactive cells of MCAF/MCP-1 were also stained with the antibody, which recognizes monocytes and macrophages. CONCLUSIONS These results suggest that MCAF/MCP-1, which is produced constantly by monocytes and macrophages and is stored in human nasal mucosa, possibly participates in the protracted histamine release from basophils and in the pathogenesis of perennial allergic rhinitis.