The localization of two carbohydrate antigens, I and sialyl I antigens, in the lungs of developing human embryos was investigated using specific monoclonal antibodies and compared with the distribution patterns of the known embryonic antigen, stage-specific embryonic antigen-1 (Lex hapten). When the future bronchi were actively developing from the bronchial buds in the lungs of 50- to 53-day-old embryos, the immature bronchial bud cells were I-, Lex+, while the fully differentiated epithelial cells of the larger bronchus were I+, Lex-. When the bronchiolar bud cells matured into bronchiolar epithelial cells in the lung of a 12-week-old embryo, the immature bronchiolar bud cells were I-,Lex+, while the fully differentiated epithelial cells of the bronchioles were I+,Lex-. Sialylated forms of the antigens finally appeared in the lungs of 18-week-old embryos, when the terminal bud cells actively proliferated and underwent the differentiation process into epithelial cells of alveoli and alveolar ducts. The immature terminal bud cells at this stage were I-, sialyl I-, Lex+, sialyl Lex-i+, while the fully differentiated alveolar epithelial cells were I+, sialyl I+, Lex-, sialyl Lex-i-. After 8 months, the flattened mature alveolar epithelial cells were strongly positive for both I and sialyl I antigens, the strong expression of which continued after birth and even into the adult stage. These distribution patterns indicate that the I and sialyl I antigens are specific markers for the differentiated type cells in each stage of development, while Lex and related embryonic antigens were specific to the immature bud cells in every stage. The above-described differentiation-dependent expression patterns of these antigens seem to be reflected in the distribution of these antigens in human lung cancers, i.e., I and sialyl I antigens were expressed in lung cancer cells more weakly than in normal lung cells, while the Lex and sialyl Lex-i were expressed in cancer cells much more strongly than in normal lung cells. This was further reflected in the serum levels of these antigens in the patients with respiratory disorders. The distribution pattern of the serum levels of these antigens in patients with lung cancers showed sialyl Lex-i greater than sialyl I, indicating that these serum antigens originated from the lung cancer lesion where sialyl Lex-i is much more dominant than sialyl I antigen.(ABSTRACT TRUNCATED AT 400 WORDS)