To elucidate the rheological difference in maternal and fetal blood by determining erythrocyte deformability, 20 pairs of mothers and newborns, and 20 nonpregnant women were studied. Erythrocyte deformability was measured by means of an electron spin resonance method. Erythrocyte deformability was dependent on the hematocrit, and there was an optimal hematocrit value at which the deformation was maximal. The hematocrit at which the deformability was maximal showed a lower value (32 to 35%) in maternal blood; conversely, a higher value (47 to 50%) occurred in fetal blood than that (40 to 43%) in nonpregnant control women. When the hematocrit of the red blood cell suspension in dextran solution was adjusted to 40% (nonpregnant blood), 35% (maternal blood), and 50% (fetal blood), the deformability of fetal erythrocytes was significantly higher than maternal blood (P < 0.05), and its value was almost similar to that of nonpregnant control women. Based on our results, erythrocyte deformability of fetal blood is much higher compared with the maternal blood, suggesting the effective oxygen supply to the fetal tissues even in the circumstances of lower oxygen tension.