In human blood incubated in vitro, the transfer of free cholesterol and lecithin from erythrocytes to serum was not related to glycolytic activity in erythrocytes or esterification of cholesterol in serum. 2. The stability of free cholesterol concentration in serum was dependent on the activity of lecithin:cholesterol acyltransferase (EC 2.3.1.43), and the stability of lecithin concentration in erythrocytes, on glycolytic activity.