Whole blood was heated for twenty minutes at 40 degrees C., 45 degrees C., 50 degrees C., and 55 degrees C. Changes in the osmotic resistance of the heat-exposed cells were then determined. The values obtained were plotted as lysis increments. This treatment revealed groups of cells of varying heat sensitivity. After heating to 50 degrees C., cell groups were therefore prepared by partitioning cells between layers of a mixture of methyl- and butyl-phthalates of known densities. Three cell groups enriched in either oldest cells, cell fragments, or cell fragments and youngest cells were obtained. These groups subjected to serialosmotic lysis tests revealed that the densest, i.e., oldest fraction, had least thermal resistance. Fractionation and osmotic resistance studies provided clear evidence of some thermal damage to erythrocytes of median age, i.e., cells which would normally be expected to remain in the circulation for another two months. Some of the possible implications are discussed.