Doubly ligated right carotid arteries of 42 rabbits were filled with cell -free serum from normal or hypercholesteremic (Triton) rabbits or with rat chylomicra. Luminal foam cells which developed in such preparations in the absence of cellular components of flowing blood were morphologically identical with those developing in the unligated left carotid. Since previous work has shown plaques formed in the absence of endothelium do not contain typical early luminal foam cells, the authors conclude these cells to be derived from the endothelial cells which formed the original intima.