Technetium-99m-Sn-phosphate colloid was prepared in the presence of polyvinylpyrollidone(PVP), molecular weight 44,000, for bone marrow imaging. Size of the colloidal particles, as determined by coulter counter, microphotographic and electron microscopic studies was 15-35 nm. The colloid preparation was checked for the presence of any soluble components by Sephadex G-25 chromatography. Scintigraphy in rabbits showed a high concentration of the colloid in the bone marrow. Tissue distribution studies in rabbits showed 26.7% of the injected dose at 1 h post-injection in the bone marrow collected from femoral shaft and head. Although liver and spleen showed considerable levels of activity, kidneys and compact bone did not show any uptake. The colloid cleared from the blood exponentionally with a first phase showing a relatively fast clearance while in the second phase it disappeared more slowly. Uptake of the colloid in human bone marrow was close to that in the rabbit and thus clinical evaluation is warranted.