We investigate the phase transition from an isotropic to a nematic liquid crystal embedded in a porous network. We model this situation by a simple lattice spin model with a field of finite strength D, but random orientation at a fraction p of randomly selected lattice sites. Monte Carlo simulations show the existence of a nematic phase with quasi-long-range order for a large D. The isotropic -> quasi-long-range nematic transition line has a tricritical point.