Optimum concentrations of singly ionized Li relative to warm protons, &eegr;≡nLi/np,w, for magnetospheric seeding experiments aimed at enhancing the electromagnetic proton cyclotron instability have been obtained analytically and numerically. This has been done by calculating the absolute maximum growth rates &ggr;max abs and enhancement F≡&ggr;max abs/&ggr;max(&eegr;-0) for the proton instability branch upon addition of cold lithium ions to an infinite anisotropic proton plasma. A necessary condition for enhancement is P≡&bgr;pAp2(Ap+1) ≪1, where &bgr;p ≡8&pgr;npKT∥,p/B0 2 and Ap≡ (T⊥/T∥)p-1. The shape of the linearly unstable spectrum strongly depends on the parameter [Ap/(Ap+1)>/(mLi/ZLimp)-1. When the condition stated above is satisfied, the growth rate and the extension of the unstable spectrum in k space may increase with &eegr;. An optimum &eegr; value is found to exist.