One serious problem encountered during microprobe studies of freeze-dried frozen sections is that it is difficult or impossible to identify healthy versus injured cells using morphologic criteria. There is a need for an independent method to identify cells which have lost plasma membrane integrity. Vital dyes such as trypan blue and erythrosin B have been used for many years in light microscopic studies of the viability of cultured cells. It is generally accepted that cells which stain positively (i.e., take up the dye) have lost plasma membrane integrity and are lethally injured. Since erythrosin B is iodinated, the authors were able to show that lethally injured cells were iodine positive, and that iodine positive cultured cells would show evidence of some of the ion shifts that are thought to occur in severe cell injury.