(We report sedimentation measurements of small colloidal particles through a nonadsorbing polymersolution. The experiment reveals that the particles "feel" the single-chain viscosity rather than thesolvent viscosity when their radius Rc is smaller than the correlation length j of the polymer solution.The particles experience the macroscopic viscosity of the polymer solution when Rc À j. In thetransition region, the particle's friction coefficient does not have the predicted scaling form. Instead,a new switch function is found to be of universal form independent of the polymer molecular weight.