Relationships between erythrocyte number, size and haemoglobin content were examined in 441 species (101 families) of mammals, birds and reptiles. Whereas the total number of red cells (RBC), the mean cell volume (MCV) and mean cell haemoglobin (MCH) showed much variation, the haemoglobin level (Hb), packed cell volume (PCV) and mean cell haemoglobin concentration (MCHC) were relatively constant in all adult mammals and birds but lower in reptiles. There was a strong positive correlation between MCV and MCH and a strong negative correlation between MCV and RBC across all species. Mammals had more, smaller erythrocytes per unit volume of blood than birds, which, in their turn, had more, smaller erythrocytes than reptiles. The findings confirm that the oxygen-carrying capacity of the blood is highly conserved in birds and mammals but is lower in exothermic groups such as reptiles. Although the significance of the observed species differences in RBC and MCV has yet to be explained, the findings have considerable evolutionary, physiological and diagnostic interest.