Isolated human corneas maintained normal corneal thickness and endothelial ultrastructural integrity throughout a three-to four-hour period of perfusion of BSS plus to the endothelial surface. By comparison, only one of six human corneas perfused with lactated Ringer's solution was able to maintain normal thickness and ultrastructural integrity. The other five corneas perfused with lactated Ringer's solution showed various degrees of endothelial cell breakdown and corneal swelling during three hours of perfusion. The results of these studies suggest that BSS plus is a better solution for maintenance of human corneal endothelium during long-term (one- to three-hour) surgical procedures.