The interaction of β-Mactoglobulin and K-casein at high temperature was studied polarimetrically. The rate of interaction was related to the genetic variant of β-Mactoglobulin present. β-Lactoglobulin B, whose thermodenaturation was faster than that of A variant, interacted more rapidly with β-casein than did the A variant. Velocity sedimentation studies indicated that characteristics of the complex were determined more by the β'-Mactoglobulin than by the K-casein. The presence of K-casein during the thermodenaturation of β-Mactoglobulin appeared to prevent the aggregation ofβ-lactoglobulin from proceeding to completion, the K-casein complexing with intermediate species and restricting the aggregation process.