ABSTRACT: The authors present theoretical calculations relating the effective diffusivity of monoclonal antibodies in tissue (D_eff) to the actual diffusivity in the interstitium (D_int) and the interstitial volume fraction phi. Measured diffusivity values are effective values, deduced from concentration profiles with the tissue treated as a continuum. By using homogenization theory, the ratio D_eff/D_int is calculated for a range of interstitial volume fractions from 10 to 65%. It is assumed that only diffusion in the interstitial spaces between cells contributes to the effective diffusivity. The geometries considered have cuboidal cells arranged periodically, with uniform gaps between cells. D_eff/D_int is found to generally be between (2/3) phi and phi for these geometries. In general, the pathways for diffusion between cells are not straight. The effect of winding pathways on D_eff/D_int is examined by varying the arrangement of the cells, and found to be slight.