A generalized model is developed to describe the transport of fluid and plasma proteins or other macromolecules within the interstitium. To account for the effects of plasma protein exclusion and interstitial swelling, the interstitium is treated as a multiphase deformable porous medium. Fluid flow is assumed proportional to the gradient in fluid chemical potential and therefore depends not only on the local hydrostatic pressure but also on the local plasma protein concentrations through appropriate colloid osmotic pressure relationships. Plasma protein transport is assumed to occur by restricted convection, molecular diffusion, and convective dispersion. In a companion paper (D. G. Taylor, J. L. Bert, and B. D. Bowen, 1990, Microvasc. Res. 39, 279-306) a simplified version of the model is used to analyze steady-state fluid and plasma protein exchange within mesentery.