Self-consistent electronic structure calculations are used to study the changes in magnetic properties (specifically the magnetocrystalline anisotropy and exchange coupling) of a Co1Pt5 superlattice induced by a grain boundary. Guided by experiment, several possible grain boundaries have been studied, and we find as a result of calculations that the intergrain exchange coupling can be reduced by up to an order of magnitude. The local anisotropy is also reduced in the neighborhood of the boundary. These calculations are aimed at developing a more realistic description of magnetic thin films that takes into account details of the microstructure and thereby provide input to micromagnetic simulations.