Using the first-principles full-potential linearized augmented plane-wave method based on density functional theory, we have investigated the electronic structure and magnetism of hypothetical MC (M=Mg, Ca, Sr, and Ba)compounds with the zinc-blende (ZB) crystal structure. It is shown that ZB CaC, SrC, and BaC are half-metallic ferromagnets with large half-metallic gaps (up to 0.83 eV). The half metallicity is found to be robust with respect to the lattice compression and is maintained up to the lattice-constant contraction of 14origin and properties of the half-metallic ferromagnetism of s-p electron systems.