During crystallization of polymorphic substances, a daughter phase may nucleate on the surface of a previously present polymorph: this crystallization scheme has been denoted as cross-nucleation. While several extensive studies exist for small organic molecules, analogous reports for semicrystalline polymers are scarce. Hereby we present the first determination of cross-nucleation kinetics for crystallizable macromolecules. The melt of isotactic poly(1-butene) was seeded with Form I spherulites through an in situ procedure. Measurements at different crystallization temperatures revealed a linear increase of Form II-on-Form I cross-nucleation density with time. In agreement with heterogeneous nucleation theories, the derived cross-nucleation rate shows an exponential dependence on undercooling.