To elucidate the characteristics of metaplastic changes of bronchial mucosa, the distribution of four epithelial antigens and two subtypes of enolases was studied immunohistochemically. The authors classified the metaplastic changes into three types: basal cell hyperplasia, stratification, and squamous metaplasia. Secretory component (SC) was detected in all lesions with stratification and in three of ten lesions of squamous metaplasia. Epithelial membrane antigen (EMA) was localized not only on the luminal surfaces but also among the stratified layers of metaplastic epithelium. No carcinoembryonic antigen (CEA) was detectable in normal epithelium, whereas almost all metaplastic lesions had both nonspecific cross-reacting antigen (NCA) and CEA immunoreactivity. In normal mucosa, basal cells were strongly positive for alpha-enolase but negative for gamma-enolase, and columnar cells expressed both enolases. In areas of metaplasia, alpha-enolase was present throughout the layer, but gamma-enolase was absent. These immunohistochemical findings suggest that metaplastic squamous epithelial cells have glandular differentiation and that several biochemical and metabolic aberrations occur during the process of metaplasia.