The purpose of the this study was to find media that supported high levels of differentiation in primary cultures of human tracheal epithelium. We tested six previously described, partially defined media and three nondefined media. Cells were grown with an air interface on porous-bottomed inserts, and differentiation was assessed from electrophysiological properties, levels of total protein and deoxyribonucleic acid, and histology, In all media, cells polarized and developed tight junctions as assessed from transepithelial electrical resistance and were better differentiated at 14 d after plating than at 7 d. The partially defined media described previously by Gray et al.(Am. J. Respir. Cell. Mol. Biol. 14: 104–112; 1996) and Matsui et al.(J. Clin. Invest. 102: 1125–1131; 1998) and an undefined medium containing Ultroser G serum substitute produced the most highly differentiated epithelial cells, as revealed by a high short-circuit current (I sc) and a ciliated, pseudostratified appearance. In other media, cells tended to be either squamous or stratified squamous, with I sc levels< 25% of those obtained with the three optimal media. Though no key factor in the composition of the partially defined media could be identified, two of the four media with high concentrations of retinoic acid produced good differentiation. In contrast, that two media with the lowest [Ca](0.11 mM produced poorly differentiated cells, as did the two partially defined media with low or no retinoic acid concentration.