Based on turbulent drag reduction characteristics of polystyrene and polyisobutylene in a pipe flow and a rotating-disk flow, respectively, a relationship between polymer concentration and drag reduction at a given Reynolds number was considered. The universal drag reduction equation of a three-parameter relationship between drag reduction and polymer concentration was modified using intrinsic concentration and intrinsic viscosity, and it was then found to be the most useful formula for correlating DR data, especially for polymer-solvent interactions in a turbulent flow. (C) 2003 Wiley Periodicals, Inc.