Bovine erythrocytes infected with Babesia bovis were analysed for parameter changes known to influence rigidity and deformability of erythrocytes. Marked increases in malonyldialdehyde were detected indicating that lipid peroxidation occurs during infection. Consequently, increases in membrane lipid, methaemoglobin and membrane-bound haemoglobin were detected. Conversely, decreases in the antioxidant vitamin E and decreases in sialic acid were also detected. The cumulative effect of these changes would be to increase erythrocyte rigidity and decrease deformability thus contributing to the microvascular stasis characteristic of acute B bovis infection.