The commonly used size of microsphere for bone blood flow estimation is 15 microns, because it has appeared to be the smallest size that is not subject to significant nonentrapment in bone. Soft-tissue studies suggest that it is microspheres of 9-10 microns or less that pass through peripheral tissues and give low calculated flows, whereas many vessels passing into and within cortical bone are less than 15 microns in diameter. We have therefore performed a comparison between microspheres approximately 15 and 11 microns in average size. Blood flows to the cortex and marrow of the tibial diaphysis, and to the skeletal muscle of the anterior compartment, were obtained in six adult New Zealand White rabbits by the reference sample technique, injecting microspheres of 16.5 +/- 0.1 microns and 11.3 +/- 0.1 microns simultaneously. The calculated cortical flows averaged 2.07 and 2.51 ml/min/100 g, respectively, and the marrow flows 26.63 and 24.92 ml/min/100 g. Mean skeletal muscle flows were 15.57 and 14.54 ml/min/100 g, respectively. There were no significant differences between the calculated flows for the two sizes of microsphere. Thus, the smaller microspheres do not appear by this method to be subject to significant nonentrapment, and they are therefore suitable for blood flow measurement in these tissues.