The proposed study uses an integrated approach to investigate both the structural and functional changes that occur in red cell plasma membrane during chick embryonic development. The biosynthesis and turnover of plasma membrane proteins, immunological characteristics and cell surface properties will be studied. We also plan to study the changes in transport properties of the plasma membrane to electrolytes (Na, K ions) water and non-electrolytes (leucine, lysine, urea, etc.). In addition, alterations in activities of membrane-bound enzymes (ATPase, adenyl cyclase) and membrane microviscosity will be studied. This systematic and comprehensive study of a system that is undergoing precisely timed and regulated changes will provide valuable information concerning the interrelationships among the structural and functional properties of the plasma membrane.