The theories that predict the increase in the modulus of elastomers resulting from the presence of a rigid filler are typically derived from Einstein's viscosity law. For example, Guth and Gold used this approach to predict how the Young's modulus of an elastomer is related to the filler volume fraction. Hon et al. have shown using finite element microstructural models that stiffness predictions at small strains were also possible. Here, microstructural finite element models have been used to investigate the modulus of filled elastomer over a wider range of strains than has been possible previously. At larger strains, finite extensibility effects are significant and here an appropriate stored energy function proposed by Gent was adopted. In this work, the effect of spherical MT-type carbon-black filler behavior was considered. Different models were made and the results were then compared to experimental measurement of the stiffness taken from the literature. It is shown that the boundary conditions of the microstructural unit cell lie between the two extremes of free surfaces and planar surfaces. Also as the filler volume fraction increases then the number of filler particles required in the representative volume to predict the actual stiffness behavior also increases. © 2007 Wiley Periodicals, Inc.