The purpose of this research is to characterize, as completely as possible, the human casein system, and to correlate any changes in this system with diet and time of lactation. Samples of mother's milk will be collected along with three-day dietary records. This milk will be defatted and the skim milk fractionated into casein micelles and whey. An ultrafiltrate will be obtained from the whey. These three fractions will be analyzed for protein and amino acids by HPLC and for mineral composition by atomic absorption spectroscopy. The natural micelles will be fractionated by differential centrifugation and analyzed as well. Monomer caseins will be characterized both chemically (amino acids, carbohydrates, P content) and physically by hydrodynamic methods (analytical ultracentrifugation and viscometry). Pure human caseins will be labelled with radioactive isotopes and used to determine if the system of natural micelles is at equilibrium and, if so, what the distribution of caseins between the various fractions might be. Protein-protein interactions will be followed with the analytical ultracentrifuge and with both u.v. and fluorescence spectroscopy under a variety of environmental conditions in order to deduce the mode of micelle assembly and the final micelle structure. The attack on the casein system by pepsin will be investigated both kinetically and structurally, by means of polyacrylamide gel electrophoresis and HPLC. A computer analysis of the dietary records will give nutrient information which may be correlated with any changes observed in milk composition. Colostrum will also be collected and the time of lactation noted in all cases so that a similar correlation between composition and this parameter may also be made. The results of this program will be an understanding of the casein system of human milk and how it functions to provide proper infant nutrition. Some insight may also be gained into how diet may affect the system and how a proper substitute may be designed for situations when an adequate supply of mother's milk is not available.