Pulmonary alveolar epithelial cells (PAEpiC), comprised of alveolar type I and type II epithelial cells, line more than 99% of the internal surface area of the lung. Type I cells are large squamous cells whose thin cytoplasmic extensions cover >95% of the internal surface area. They contain aquaporins and exhibit the highest osmotic water permeability of any mammalian cell type. Type II cells, which cover 2-5% of the surface area, produce, secrete, and recycle pulmonary surfactant. Type II cells contain Na+-, K+-ATPase and amiloride-sensitive epithelial Na+ channels. The currently accepted hypothesis is that Type II cells maintain pulmonary fluid homeostasis by regulating active Na+ transport in the lungs, whereas Type I cells are "inert" cells that provide solely a barrier function, rather than having active functions. Recent studies indicate that Type I cells are also important in regulating ion and fluid transport. HPAEpiC-a from ScienCell Research Laboratories are isolated from human adult lung tissue. HPAEpiC-a are cryopreserved at P0 and delivered frozen. Each vial contains >1 x 106 cells in 1 ml volume. HPAEpiC-a are characterized by immunofluorescence with antibodies specific to cytokeratin-18, and/or cytokeratin-19. HPAEpiC-a are negative for HIV-1, HBV, HCV, mycoplasma, bacteria, yeast and fungi. HPAEpiC-a are guaranteed to further culture under the conditions provided by ScienCell Research Laboratories; however, HPAEpiC-a are not recommended for expanding or long-term cultures since the cells would differentiate to become type I alveolar epithelial cells immediately after plating and type I alveolar epithelial cells do not proliferate in culture.