A modified cell-model is proposed to describe the process of simultaneous homogeneous and heterogeneous nucleation. The modification consists in considering that the seed particles are randomly distributed in the system and in assuming that homogeneous nucleation can only occur in those places where the interparticle distance is large. The model predicts the relative extent of homogeneous and heterogeneous nucleation as a function of the operating conditions (concentration of seed particles and monomers and seed-to-nucleus size ratio). The calculations are in very good agreement with the results obtained in liquid-phase nucleation experiments.