Highly purified populations of alveolar epithelial cells (type II pneumocytes) were isolated from human lung specimens. These cells were characterised histochemically, by demonstrating the presence of intracellular alkaline phosphatase, and morphologically, by electron microscopic demonstration of lamellar bodies and microvilli. Expression of the epithelial glycoprotein HEA-125, of MHC class I and class II (HLA-DR, -DP and -DQ) antigens and of the intercellular adhesion molecules ICAM-1, VCAM-1, LFA-3 and B7 was quantified by flow cytometry. Comparison was made between the expression of these molecules by isolated type II cells and by alveolar epithelium in normal human lung tissue after immunocytochemical staining of frozen sections of donor lung. Isolated type II pneumocytes expressed HEA-125 and class I MHC molecules and the class II MHC molecules HLA-DR and -DP; HLA-DQ was not detected. The intercellular adhesion molecule ICAM-1 was expressed constitutively at low levels but there was minimal expression of VCAM-1, LFA-3 and B7. It was not possible to differentiate type II cells from the predominant type I pneumocytes on frozen sections. Alveolar epithelium expressed HEA-125, class I MHC antigens, the class II molecules HLA-DR, and -DP and the intercellular adhesion molecule LFA-3. Expression of the adhesion molecules ICAM-1, VCAM-1 and B7 was variable. As with the isolates, HLA-DQ was not observed on alveolar epithelium. In conclusion, a reproducible method for the isolation of pure populations of human type II pneumocytes has been developed. These cells were not damaged by the isolation procedure. It is not known whether alveolar epithelium can present antigens to T lymphocytes.(ABSTRACT TRUNCATED AT 250 WORDS)