In this paper, we study a discrete-time version of the Lucas model with externality but without physical capital. We give conditions for which the optimal human capital sequences are increasing. When the instantaneous utility function is isoelastic and the production function is Cobb-Douglas, we prove that the optimal human capital sequences grow at constant rate. Moreover, there exists a unique equilibrium which, under an additional assumption on the human capital technology, is also the unique competitive equilibrium.