Recently, sp half-metallic ferromagnetism in alkali metal/alkaline earth pnictides (I/II-V) with zinc blende structure was predicted by first-principles calculations (Sieberer et al 2006 Phys. Rev. B 73 024404). To search for half-metallic ferromagnets with a more stable structure in alkali metal pnictides MX (M  = Li, Na, K; X =  N, P, As), we apply here first-principles all-electron and pseudopotential-method-based density functional theory to investigate the electronic and magnetic properties of these compounds with rocksalt structure. Five rocksalt compounds are found to be half-metallic ferromagnets with a magnetic moment of 2.00 μ(B)/f.u., and they are more stable energetically than the ones with zinc blende structure, which makes them more promising candidates for the possible growth of half-metallic films or ultrathin films on suitable semiconductor substrates. Furthermore, we show that both lithium- and nitrogen-terminated (111) surfaces of rocksalt lithium nitride retain the bulk half-metallic characteristic, and the surface atomic magnetic moments are enhanced compared to the bulk values.