To obtain additional information relevant to polymer behavior near human red blood cells (RBC), sedimentation and electrophoretic mobility behavior of RBC in the same suspension were determined for cells in various isotonic solutions of 10.5, 74 and 519 kDa dextran. Regardless of medium viscosity or dextran molecular mass, RBC sedimentation rate could be predicted via the Stokes' equation. Conversely, the effects of medium viscosity on RBC mobility were markedly overestimated by the Helmholtz-Smoluchowski relation, with the error increasing with dextran molecular mass. These results confirm and extend prior electrophoretic mobility studies, agree with the concept of polymer depletion near the RBC surface, and lend strong support to a "depletion model" mechanism for dextran-mediated reversible RBC aggregation.