The Cell Transit Analyzer (CTA) is now being used widely in clinical hemorheology. Most of the data obtained by CTA are limited to human blood, although the CTA has an important potential to be used in experimental studies on animal models. However, behavior of red blood cells (RBC) from various species might be different in CTA, Eight parameters reflecting different aspects of cell passage through pores with 5 mu m diameter and 15 mu m length were determined or human, guinea pig, dog, rabbit, rat, mouse and sheep RBC, together with instrument precision and biological variation, These parameters have a wide range when measured in different species and correlate with cell volume. Sensitivity of these parameters to the glutaraldehyde-induced alterations in IBC deformability was not same for different laboratory mammals, The main reason for this difference seems to be related to the cell size and thus sensitivity might be significantly limited if 5 mu m pore-size filters are used to test the smaller RBC. The results of this study may help in designing experimental studies on laboratory mammals using the CTA.