β-lactoglobulin (BLG) is the major whey protein in many ruminant and non-ruminant milks, and, although it has been studied extensively, its function in vivo remains a mystery. Several of the properties of this protein which may provide information on its function are: the distribution of BLG amongst species, and structural homology of BLG to serum retinol binding protein and two bilin binding proteins, its inclusion in a superfam-ily of transport proteins, the nature of the hydrophobic binding sites, and the pH-induced reversible conformational change (the Tanford transition) which bovine BLG is known to undergo over the physiological pH. These are examined in this thesis. The preliminary crystal structure of bovine BLG lattice Y at pH 7.8 (space group B22t2: a= 55.7A, b= 67.2A, c= 81.7A) had been determined previously. This was improved by the inclusion of more data, and refinement by manual model-building using the molecular graphics program FRODO, least-squares, energy minimization and molecular dynamics. The protein electron density map generated at 2.8A resolution was used to examine the positions of the antigenic regions, and the environments of specific amino acids. A comparison of this structure, with the lattice X structure (space group PI: a= 37.8A, b= 49.6A, c= 56.6A, a = 123.4°, ft = 97.3°,T = 103.7") determined at pH 6.5 [141] was undertaken, so as to offer a molecular explanation for the conformational change which occurs between these pHs in solution. This transition was known to involve an anomalous carboxyl, and the identity of the residue is discussed. The free cysteine, phenylala-nine and tyrosine residues were also implicated. Solution studies, using the techniques of polarimetry, circular dichroism, and tryptophan fluorescence, were carried out to confirm the nature of the Tanford transition. The inclusion of BLG in a superfamily of transport proteins, and its isolation with fatty acid bound, indicated that its function could be that of a small molecule carrier. The binding of retinol, p-nitrophenyl phosphate and biliverdin to BLG was investigated, to elucidate the nature and importance of the two proposed hydrophobic binding sites - the cavity of the ^-barrel, and the external channel near the a-helix. The relationship between ligand binding to BLG and the Tanford transition is discussed, and a function for this protein is proposed.