Turbulent drag reduction induced by dilute solutions of both water-soluble poly(ethylene oxide) (PEO) and oil-soluble polyisobutylene (PIB) under turbulent flow in a rotating disk apparatus was investigated. The concentration dependence of drag reduction for these systems was shown to obey an empirical drag reduction equation which has been previously applied to describe flow in circular pipes. Linear correlation between polymer concentration and drag reduction for different molecular weights of PEO and PIB was also obtained. The intrinsic concentration [C] was found to be very useful in normalizing the drag reduction data for different types of polymers, polymer molecular weights, concentrations, and rotational disk velocities. Explanation of the drag reduction phenomenon on a molecular level can be obtained from intrinsic drag reduction and [C].