Blood from non-inbred obese-hyperglycaemic ob/ob-mice or normoglycaemic controls was fixed in glutaraldehyde and embedded in plastic on glass slides. In vertically oriented red blood cells (RBCs) the diameter, central thickness, and toroidal thickness were measured at the diametrical cross section. For each RBC, the area, volume, and cross-sectional profile were calculated and used to analyze the mechanical properties of the corpuscle. In both types of mice, the diameter correlated positively with the central thickness and negatively with the toroidal thickness, suggesting a variation not only in size but also in biconcavity; the smaller the diameter, the more biconcave the disc. However, ob/ob-mouse RBCs were both larger and more biconcave than those in control mice. These differences in size and shape are suggested to explain why ob/ob-mouse RBCs exhibit a decreased deformability in filtration experiments.