The three-dimensional shape of erythrocytes is assessed by light microscopy using a simple technique to view glutaraldehyde-fixed erythrocytes. The technique and its standardization is described. Using this method the erythrocytes of adults have been compared to those of full-term and premature infants. In adults, the majority of erythrocytes are bowl-shaped at 37 degrees C and disc-shaped at room temperature. Newborn infants have significantly more bowl-shaped cells as well as significantly more irregularly shaped cells. Erythrocyte differential counts have been performed on a large series of patients to determine the normal range for each cell shape in adults, full-term, and premature infants. It is suggested that the study of erythrocyte morphology in haematologic disease in newborn infants requires three-dimensional assessment and quantitative data of normal values.