Languages with parametric polymorphism provide the compile-time guarantee that programs behave uniformly regardless of the types they are instantiated with. In such languages, this parametricity guarantee forms the basis of data abstraction and makes possible Wadler’s “theorems for free!” In this talk, I’ll show how we can extend System F’s parametricity guarantee to a Matthews-Findler-style multi-language system that combines System F with a Scheme-like language by use of dynamic sealing, which enforces data abstraction by using uniquely generated keys to seal values that should be kept opaque. While the use of sealing for this purpose has been suggested before, it has not previously been shown to preserve parametricity. Our proof employs a cross-language, step-indexed logical relation that uses possible worlds to capture the semantics of seal generation. Using this possible-worlds model, we can show two results: first, that System F’s parametricity guarantee is preserved when interoperating with Scheme, and, second, that parametricity is preserved in a Scheme-only setting that interacts with System F only to implement a polymorphic contract system. (This talk describes current research being done in collaboration with Amal Ahmed and Jacob Matthews that expands on the work presented in their ESOP 2008 paper of the same title.)