Adult and newborn infant blood viscosity have been compared, taking into account not only the hematocrit, but also the type of red blood cells (fetal or adult) in the circulation and the plasma viscosity. At all shear rates studied, the viscosity of the adults' blood was higher than that of the newborn infant. At shear rates of 11.5 and 46 second-1, an increase in the hematocrit influences the viscosity of neonatal and adult blood similarly. At 115 and 230 second-1, the rise in hematocrit was associated with a greater increase in viscosity in the presence of fetal red blood cells, probably because of their lesser deformability. Plasma viscosity was 1.18 +/- 0.17 centipoises in the newborn compared to 1.36 +/- 0.10 in the adult group (P less than 0.001). The relative apparent viscosity (apparent viscosity/plasma viscosity) was higher in the neonate at a hematocrit of 65% (P less than 0.05). In normal conditions, blood viscosity is lower in the neonatal period because of a lower plasma viscosity.