The author studied on the in vitro incorporation of I-(14)C-acetate into total lipids and total fatty acids of platelets. The results are summarized as follows. I. The pattern of incorporation of radioactivity into fatty acids of platelets obtained from normal subjects. a. It was ascertained that Platelets synthesized lipids, particularly fatty acids and fatty acid synthesis was considered to be via both mitochondrial and malonyl CoA pathways in platelets. b. Platelets showed usually a significant decrease in percent (14)C incorporation into oleic acid from I-(14)C-acetate as compared with liver, adipose tissue and leucocytes. c. Platelets showed an increase in percent (14)C incorporation into nonesterified fatty acid and a decrease into triglyceride as compared with liver, adipose and leucocytes. d. Every fatty acid synthesized by platelets was selectively esterified to phospholipid and triglyceride. e. The percentage of radioactive oleic acid in triglyceride fatty acids was more increased than that in nonesterified fatty acids. 2. The pattern of incorporation of radioactivity into fatty acids of platelets obtained from patients with arteriosclerosis, hyperlipidemia and diabetes mellitus. a. The (14)C incorporation into total fatty acid was increased markedly in patients of hyperlipidemia and obesity. b. The (14)C incorporation into cholesterol also was increased in these patients. c. Fatty acid synthesis via malonyl CoA pathway was decreased and alternatively fatty acid synthesis was increased via mitochondrial pathway. d. Increased (14)C incorporation into oleic acid was observed and considered to be due to the enhancement of monounsaturation of stearic acid.