The distribution and density of metachromatic cells (MCC) and mast cells containing chymase plus tryptase (MCTC) or tryptase alone (MCT) were studied in the nasal mucosa by dye-binding methods and immunohistochemical analysis. Biopsies were obtained from 17 subjects with birch pollen allergy before and during the peak season and from nine healthy controls. Six patients were treated with an intranasal glucocorticosteroid before and during the season in an open study. Hay fever patients, even when asymptomatic, showed signs of mast cell system activation, exhibiting an increased number of mast cells in the nasal epithelium. Basophils, lacking immunohistochemically detectable tryptase, were not a major component of the mast cell response. MCT, most conspicuous in the epithelium, were found to be the most frequent mast-cell type in the nasal mucosa of allergic, but not of normal, subjects. Only 33% of the epithelial, but 90% of the stromal, immunopositive cells in the atopic mucosa before as well as during the season were MCC. Intraepithelial MCT thus displayed a low capacity to stain metachromatically, indicating a relative deficiency of the glycosaminoglycan (heparin) component of the granules. Intraepithelial mast cells also appeared to be markedly sensitive to steroid treatment and aldehyde fixation. The findings suggest that the lack of chymase, the characteristic feature of MCT, may reflect a functional activation of the mast cells, rather than a stable phenotypic differentiation related to anatomic site.