The organ distribution of very low density lipoproteins, with known hematopoietic cell growth inhibitory effects, was studied in the rat. Animals received intravenous injections of 125I-labelled VLDL and tissue uptake was monitored over 4 days. Uptake into 8 organs was studied using a technique that excluded blood associated radioactivity. While we demonstrated that the liver predominated in tissue uptake of radioactivity on a total organ basis, bone marrow had the greatest 125I-VLDL uptake per gram of tissue, six times greater for bone marrow than liver. Thus we have demonstrated that the bone marrow is a very active organ in terms of uptake of VLDL. This suggests that the in vitro inhibitory effect of rat VLDL on bone marrow cell proliferation may have physiologic significance in the in vivo regulation of marrow cell proliferation.