The biocompatibility of poly(propylene oxide)-segmented nylon610 (PPO-Ny610), poly(ethylene oxide)-segmented nylon610 (PEO-Ny610), poly(ethylene oxide)-segmented nylonM10 (PEO-NyM10), and poly(ethylene oxide)-segmented nylon69/M10 (PEO-Ny69/M10) hollow fibers were investigated in terms of the transient leukopenia by the extracorporeal circulation in a rabbit. PPO-Ny610 and PEO-Ny610 hollow fibers showed that the minimum leukocyte counts during the circulations were >80% against the initial count of leukocyte. These results indicate that these polymers have good blood compatibility. In PEO-NyM10 and PEO-Ny69/M10, the remarkable decreases of the leukocyte count were observed and the minimum counts were in the range of 45-50%. From the evaluation results of homo nylons (Ny610 and NyM10) hollow fibers, the low blood compatibilities observed in PEO-NyM10 and PEO-Ny69/M10 are not attributed to the chemical structure of the nylon blocks.