The effects of lithium and heavier ions (i.e. having specific charge Zj e/mj smaller 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; Ωj, 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 is smaller than a critical value A pc (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 ion and warm proton particle densities, respectively. There is an optimum value of η for maximum enhancement of the instability. (ii) If Ap > Apc, the unstable spectrum in Wr 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.