Creep compliance was used to determine the effects of the addition of a-, @-, and K-casein, and Na-caseinate on the viscoelastic properties of skim milk curd. The results of all measurements can be represented by a six-element mechanical modet. Addition of a-and @-casein. and Na-caseinate ( I .80giL) to raw skim milk reduced the instantaneous modulus of rigidity and final viscosity of the curd, while K-casein addition at the same level increased both viscoelastic parameters. Shielding of K-casein and depletion of serum Ca" ions by a-and @-casein is thought to have caused the reduction of curd rigidity and viscosity. Subsequent experiments indicated that the addition of @-casein before and after rennet hydrolysis produced diflerent curd strength with the latter producing a stronger curd.