Although the role played by cold ions on the electromagnetic proton-cyclotron instability has been thoroughly investigated, there are some aspects which deserve further attention. Moreover, since the presence of ion cyclotron waves in the magnetosphere has been experimentally established it is important to have a full understanding of the role played by cold ions. Thus, it is shown here that the main effect of cold ions on the proton-cyclotron instability is to shift the unstable spectrum. If the only cold species are protons, then the shift of the spectrum is always towards lower frequency values, i.e. away from the marginal mode. If in the absence of cold protons the maximum convective growth rate lies far from the marginal mode, the only effect of cold protons is to shift the spectrum further away from the marginal mode. However, if the maximum convective growth rate in the absence of cold protons lies close to the marginal mode, then the shift of the spectrum leads also to a large amplification. The reason for this behaviour lies in the fact that as the thermal anisotropy decreases, the frequency range of the unstable spectrum also decreases, stabilizing high frequency waves. Since unstable higher frequency waves occur in the absence of cold protons, these are the first to be stabilized. The observed amplification due to cold protons is the result of comparing waves which have suffered some stabilization with waves which, due to the shift of the unstable spectrum, have not yet been affected by the stabilization which follows the reduction of the thermal anisotropy. A consequence of these results is that, since the thermal anisotropy decreases as the instability develops, the instability lasts longer in the presence of cold protons than in their absence. If more than one cold species are present, the situation is more complex but the basic mechanism is the same. It is conjectured that the catalytic role played by cold He + ions in the generation of ion cyclotron waves in the magnetosphere consists of shifting the unstable spectrum away from a forbidden or sta- bilization region.