Diffusion can often be the limiting factor in determining the rate of a liquid-phase process. On the basis of absolute rate theory, Metzner and co-workers developed theoretical expressions for the prediction of diffusivities in viscous liquids and polymer solutions. Those equations are summarized here, and this approach is modified for the case of dilute polymeric solutions, including biological ones, in order to gain insight into the mechanisms involved in altering diffusion rates. Increasing the diffusivity may be especially beneficial in biological systems, where small changes might have a significant effect. For example, it is suggested that increasing the diffusivity of oxygen could alter (I) the respiration rate of microorganisms in culture and (2) the rate of oxygen consumption in animals, and data for these are presented.