The forces of interaction between a flat poly(tetrafluoroethylene) (PTFE) surface and gold spheres (of radii 3-8 mu m) were measured as a function of apparent surface separation for different intervening media, For air, fluorinated alkanes, and polar liquids the interaction between the surfaces was found to be attractive, With intervening liquids of low-polarity the interaction was found to be repulsive, This repulsion is attributed to a negative composite Hamaker coefficient leading to van der Waals repulsion.