This paper presents the results of a computer simulation of the modifications produced in the whistler instability by the addition of varying amounts of cold plasma to an infinite homogeneous warm (w) plasma with parameters typical of those observed in the equatorial magnetosphere beyond the plasmapause (β ≡ 4πnwkT∥,w/B0² = 0.8 and (T⊥/T∥)w = 2). Specifically, the particle-in-cell simulation method for electromagnetic interactions in infinite homogeneous magnetized plasmas has been used. The experiment has been performed for three values of relative cold (c) plasma concentrations, nc/nw = 1, 2, and 5; prior results for nc/nw = 0 are also discussed. The results of the experiment confirm the basic predictions of theoretical linear stability analysis for the prescribed conditions. In the linear stage of evolution the addition of cold plasma (1) increases the instability range in k space and thus enables resonant interactions of electrons having lower parallel velocities and (2) displaces the k location of the maximum rate of growth toward larger k values. Additional quasi-linear effects indicate that as a consequence of particle diffusion in velocity space the thermal anisotropy ratios (T⊥/T∥)w decrease continuously, the result being a decrease of the larger k mode growth rates and a displacement of maximum wave amplitude toward smaller k values. All the systems relax to quasi-stationary states with approximately the same (T⊥/T∥)w value, ≃1.35 (which linearly should be unstable). However, in this stage, the larger the initial nc/nw ratio, the larger the total energy in the electromagnetic waves.