Using electron microscopy (EM), it was examined whether cryopreservation with HES causes shape changes of erythrocytes. Each of 11 erythrocyte suspensions (Hct = 40; HES 200,00/0.5/12.5%; 60 mM NaCl) was separated into 40-ml samples, cooled down to -196 degrees C and finally stored. In addition, 11 samples were stored at -80 degrees C for 3 months. The preparation for EM was done immediately after thawing or in case of native cells shortly after donation. On EM micrographs, there was no visible difference between native and cryopreserved erythrocytes. In every case the preparations showed normocytes, either as single cells or having attracted other ones, forming rouleau. Packed cells were attached tightly to each other without any gap in between. The tangent count method neither revealed an excess of convexity nor of concavity. The erythrocyte membrane looked normal, and the cytoplasmatic space was filled with electron-dense material (hemoglobin) homogeneously; Heinz bodies were not seen. Scanning microscopy portrayed native as well as cryopreserved cells as discocytes with the characteristic bioconcave resting shape of human erythrocytes. It is concluded that cryopreservation of erythrocytes with HES does not cause shape changes. Therefore, a sequestration into the reticulo-endothelial system (RES) by means of identification of morphological abnormalities may not be expected.