Lots of problems remain unsettled as to the developmental mechanism of atherosclerosis despite efforts of many excellent researchers. Various sorts of factors, such as endocrine, dietary, genetic and acquired habitual, are supposed to take parts resulting in the abnormal metabolism of lipids, formation of thrombus, abnormal discharge of angiospastic substance, changes in blood pressure as well as structural alteration of the vascular wall. No satisfactory physiopathological studies have been carried out on the developmental mechanism of atherosclerosis. Attention has been payed to the relationship between stress and the development of atherosclerosis develops. Seifter and his colleagues demonstrated the formation of lipid mobilizer, a substance to increase serun lipids, in the serum of animals given cortisone or exposed to cold. The author has attempted to clarify the relatioship between stress and the developmental mechanism of atherosclerosis with special attention to the lipid mobilizer (LM). The present paper is to report the effects of LM on the development of atherosclerosis in the rabbits fed on lanolin. Material and Methods 1) Adult mal rabbits of the same age, ranging from 2.5 to 3.5 Kg in body weight, were used for the experiment. 2) Purification of lipid mobilizer. Intramuscular injection of cortisone acetate, 5 mg per kilogram, was performed on the adult rabbits that had been kept without feeding for 24 hours ; 7 hours later, the whole blood, being added with 10% sodium citrate, was obtained. The serum was separated and dialysed for 48 hours in running water, and then dialysed for 48 hours in Aq dest. The supernate was concentrated under slightly decreased pressure at 68°C, and then dried by freezing for the stock. The stock material was dissolved in physiological saline solution every time for use. 3) The rabbits were divided into 4 groups. Group 1 (control group) : Fed on pellet diet (RC 5), 200 g per day, and injected with physiological saline solution. Group 2 : Fed on the mixture of lanolin (20 g) and pellet diet (200 g), and injected with physiological saline solution. Group 3 : Fed on pellet diet, 200 g per day and injected with LM 5 mg per kilogram of body weight, every day.