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Abstract: The activity of Kluyveromyces lactis β-galactosidase increased when it was measured in the presence of either β-lactoglobulin or bovine serum albumin, whereas α-lactalbumin showed no effect on enzyme activity. Heating β-lactoglobulin further increased β-galactosidase activity. Affinity chromatography assays demonstrated that lactase bound specifically to β-lactoglobulin, resulting in enzyme activation. Heating β-lactoglobulin in the presence of lactose resulted in a reaction between the protein and the sugar (lactolation), which diminished the binding capacity of the protein to the enzyme and therefore its activating effect. It was concluded that β-lactoglobulin raises lactase activity through two different mechanisms, one of which depends on the release of sulfhydryl groups by heat treatment from the denatured protein, and the other a result of the ability of the native protein to bind the enzyme. [Copyright &y& Elsevier]