Paired human corneas (48) were preserved at 31 degrees C for different periods of time up to 122 days in a modified MEM tissue culture medium free of or supplemented with 8% dextran T500. Cell density and vitality of the endothelial cells were analysed using morphometry, light microscopy after trypan blue staining, followed by sucrose provoked swelling and fine structural observations. Culturing for periods up to 7 weeks resulted in an endothelial cell loss of 11% of dextran-blue and of 15% in dextran-supplemented medium. For preservation periods between 9-13 weeks, these values were 43% and 36% respectively. The differences between dextran-free and dextran-supplemented preservation media were not significant. As deduced from trypan blue staining, sucrose provoked swelling and fine structural observations; the remaining cells were vital. Dextran was taken up by the endothelial cells. The practical consequences of endothelial cell loss and the toxic effects of dextran are discussed in relation to penetrating keratoplasty.