Our previous model for simultaneous homogeneous and heterogeneous nucleation, which was successfully tested for silver particle generation by a chemical reduction method, is extended to the case of slow reaction rate. Nucleation in a system containing seed particles in the case where the reaction rate is slow occurs according to the following rule: homogeneous nucleation is predominant when n(P)/n(0)* (n(P) is the seed number concentration, and n(0)* the number concentration of homogeneously nucleated particles without seed particles), and heterogeneous nucleation is predominant when n(P)/n(0)* < 1. This theoretical prediction is in good agreement with the experimental results obtained for zinc sulfide particles generated by homogeneous precipitation.