The present study was performed to find out whether the type II alveolar epithelial cell or its precursor (an approximately cuboidal cell lacking multilamellar bodies) is present before the twentieth week of human gestation. For this purpose we used an antibody, SALS-Hu(E), which recognizes the human type II cell on the basis of surfactant-associated proteins. Application of SALS-HuE (by indirect immunofluorescence) to acetone-fixed frozen sections of fetal lung tissue gave a distinct staining of the cuboidal or low columnar epithelial cells lining the end-pieces of the tubular system of fetal lung (initially only a few): this staining started around weeks 10 to 12 after conception. Around week 16 some of the labeled epithelial cells appeared to be rather flat and by week 19 a combined cellular and linear fluorescence pattern was seen. Columnar epithelial cells of the prospective bronchial portion did not show this specific staining. Our results indicate that the type II cell or its precursor cell is indeed present in the pseudoglandular period of human lung development, i.e., starting around the tenth to twelfth week. This cell type lines the acinar tubule, the basic structure of the pulmonary acinus. Transformation of this cell type into the type I alveolar epithelial cell seems to start in week 16.