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SynopsisThe conformation of (3-casein A in the monomeric and thermally aggregated states has been investigated by a range of techniques. P-Casein exists as a monomer in solution a t 4°C and a t concentrations up to a t least 3 g/dl. The molecule is flexible and exhibits a lot of segmental motion, but its secondary structure is not wholly random coil; about one-third of the polypeptide chain is ordered and the likely locations of these regions are discussed. The radius of gyration, representing the time-average distribution of the flexible chain, is 46 A. Increasing temperature leads to aggregation of the P-casein molecules. The degree of association is very sensitive to experimental conditions, and under our conditions a 14-mer exists a t 20°C. The aggregate is spherical with a radius of about 100 A. The interior of the aggregate is relatively disordered, and the P-casein molecules remain in a largely flexible, hydrated conformation. The volume restriction of the protein molecules which occurs on association leads tvsome immobilization of the hydrophobic C-terminal region, which is packed toward the center of the aggregate.