Since Duguid suggested that atherosclerosis represents essentially the organization of mural thrombi, there have been many attempts to produce the disease experimentally by damaging the arterial wall. A single injury to the inner lining of an artery causes lipid-free lesions, composed of smooth muscle cells and collagen, covered by endothelium. Previously, we reported the development of atherosclerotic lesions in normolipemic rabbits as a result of repeated or continuous intimal injury by an indwelling aortic polyethylene catheter. However, it was difficult to control the location or duration of the intimal injury. The present investigation was designed to produce repeated endothelial injury in a defined segment of rabbit carotid artery. Sixty-two rabbits received injections of either lymphocytotoxic-positive (LP) or lymphocytotoxic-negative (LN) human serum into a segment of left carotid artery. Autologous rabbit serum was injected into the right carotid artery as a control. Eight rabbits received a single injection of LP and were killed 4 weeks latermforty-two rabbits received injections of human serum at weekly intervals, for a maximum of four injections, and were killed 1 week after the last injectionmthirty-two of 42 rabbits received repeated injections of LP; 10 received repeated injections of LN. Raised, lipid-containing lesions were present in 21 of 26 rabbits receiving four repeated injections of LP. No, or very minimal (fewer than three cells thick), intimal thickening was found in the 10 LN rabbits and in all control right carotid arteries. In eight rabbits receiving one injection of LP, fibrous intimal thickening without lipid accumulation, fatty streaks, and edematous plaques were found. Electron microscopy of arteries from 12 rabbits sampled at 1,5, and 60 minutes after exposure to LP indicated that the initial damage was loss of endotheliummthe results consistently showed lipid in raised, thrombus-covered (non-reendothelialized) lesions. Nonraised, endothelialized lesions did not show lipid. These findings support the belief that atherosclerosis occurs in response to repeated endothelial injury.