Micropore filtration of dilute red blood cell (RBC) suspensions is a widely known method for determining red blood cell deformability. Use of this method for cells from various laboratory animal species does require considering the effects of the cell size to pore size ratio and of suspension hematocrit. In general, previous animal studies have utilized 5% hematocrit suspensions and five micron pores, and thus conditions similar to human clinical laboratory practice. However, when used for repeated sampling from small laboratory animals or for parallel multiple samples from different sites in large laboratory animals, the volume of blood sampled and hence the hematocrit of the test suspension may be limited. Our results indicate that hematocrit levels yielding stable values of RBC pore transit time are pore size and species specific: three micron pores=2~5% for dog and 3~5% for rat; five micron pores 3~5% for dog and 1~5% for rat. An analytical approach using a common expression for calculating transit time is useful for determining the sensitivity of this time to hematocrit alterations and hence to indicate hematocrit levels that may be problematic.