The effects of lithium and heavier ions (i.e. having specific charge Zje/mjsmaller than the proton's), on the electromagnetic cyclotron instability in warm anisotropic (bi-maxwellian) proton plasmas, are analytically and numerically investigated. It is found that the occurrence of resonances for modes with ωr= Ωj. (ωr, real wave frequency; Ω.j ion gyrofrequency), which produce a cut-off in the phase velocity, leads to the possible existence of two different situations, (i) If the proton thermal anisotropy Ap = (T⊥/T||)p—1 is smaller than a critical value Ac p(which depends on the ion specific charge), the resonance and therefore the 'stop band' occurs outside the unstable range. The maximum growth rate, as well as the instability range in k space, may increase with the relative ion concentration ηj≡Zjnj/np, w (njnp,w: ion and warm proton particle densities, respectively). There is an optimum value of η for maximum enhancement of the instability, (ii) If Ap > Ac p the unstable spectrum in ωr space is divided by the 'stop band' into two distinct regions. In the higher-frequency region, the maximum growth rate, as well as the width of the unstable spectrum, decrease with increasing relative ion concentration. For η> ηc the instability is completely suppressed. In the lower-frequency region, however, the maximum growth rate and the extension in k space may increase with η.