A clear relationship was found to exist when the echoicity of whole blood under different flow conditions was determined and related to its corresponding ultrasonic backscattering properties. The results indicate that the echoicity of whole blood is shear rate and species dependent. Echoicity of porcine whole blood was found to decrease as the shear rate was increased, whereas the echoicity of bovine whole blood was found to be shear rate independent. At the same shear rate and hematocrit, the echoicity of human and porcine whole blood was found to be higher than that of bovine whole blood. These observations can be readily understood when red cell aggregation is considered. In imaging porcine whole blood under steady laminar flow, under certain conditions a hypoechoic region was observed to appear near the center of the flow conduit. The behavior of this hypoechoic region and the mechanisms responsible for its appearance are not entirely clear at present.