Linear viscoelastic properties of dispersions of vesicles are investigated by improving the cell model. The effective viscosity is defined as the response function of the average stress to the average velocity gradient. By noticing the difference between the average velocity gradient and that of the unperturbed flow, which is neglected in the previous theories on the cell model, the effective viscosity can be put into the form analogous to the Clausius-Mossotti equation for the permittivity of a dielectric as proposed by Bedeaux. The formula is applied to the calculation of the effective complex viscosity of nondilute dispersions. ©2000 American Institute of Physics.