We have provided immunocytochemical evidence that the CEM replacement medium supports the appropriate membrane domain expression of Na,K ATPase when the cells are grown on Millicell chambers. Excellent TERs and robust expression of bestrophin are also observed. This combination of features was not observed when other, standard culture media were used. The results suggest that, under these conditions, cultured human RPE develops a highly differentiated and functional polarity appropriate for the in vitro modeling of RPE in vivo function.