This paper describes an investigation of the deformation of the Co binder in a WC-Co hardmetal, based on the simulation of void growth in the microstructure. The growth of a periodic arrangement of voids is analysed over a range of macroscopic strain slates. The modelling involves a detailed description of the microstructure incorporating a physically based, finite deformation, rate dependent, crystal theory of plastic slip. The theory is implemented using the finite element method. The effects of void shape, overall stress state and binder layer thickness on void growth behaviour are assessed. Results are also presented showing the effects of varying the lattice orientation as well as slip system arrangement. Comparison is made with the predictions of phenomenological constitutive theories, J(2) flow theory with isotropic hardening and J(2)-based Gurson theory allowing for dilatant plasticity, and the incompressible, isotropic power law analytical model of Duva containing a dilute concentration of spherical voids, to assess the importance of detail in the material description. Copyright (C) 1996 Elsevier Science Ltd.