Abstract We report a first-principles study of the hypothetical RbN and CsN compounds. The spin-polarized calculations indicate that these materials are half-metallic ferromagnets and exhibit an integer magnetic moment of 2μB. The most important property in these alkaline metal nitrides is that the half-metallicity is originated from the polarization of the p-N orbitals not present in the conventional transition metal or rare earth nitrides. We find also that the half-metallicity is maintained in the zinc-blende structure on a wide range of lattice constant. This offers the possibility to growth such materials on various semiconductor substrates.