For four consecutive weeks, 61 rabbits received weekly injections of lymphocytotoxic-positive human serum into the left carotid artery and of autologous serum into the right carotid artery as a control. Serum cholesterol and serum triglyceride levels were measured before the study, in the second and fourth weeks of the study, and weekly thereafter. The results show that repeated intimal injury caused raised, lipid-containing thromboatherosclerotic lesions and that there was a consistent regression to lipid-free fibromusculoelastic plaques from the first week after completion of the injection regimen to the fourth week. Apparently, regeneration of an intact covering cell layer resulted in the elimination of lipid deposits from raised lesions, resulting in lipid-free fibromusculoelastic plaques. In addition, fatty streaks were observed to occur during regression. A statistically significant rise in serum cholesterol level during the phase of progression of lesions and a subsequent fall during regression were observed.