The half-metallic NiMnSb compound is recently attracting renewed interest for its application in devices making use of spin polarized charges injected into conventional semiconductor. For technological purposes it is desirable to design interfaces where the peculiar half-metallic character of NiMnSb is not degraded.We focus here on the NiMnSb/GaAs(0 0 1) heterojunction that we investigate by accurate pseudopotential density functional—local spin density calculations. We find in general that the half-metallicity is lost when NiMnSb is joined with the semiconductor, even at the interface with a mixed (Mn,As) plane which is one of the most promising for maintaining the desired half-metallicity. In this case, however, the effect is localized just on the interfacial atoms. Electronic and magnetic bulk properties are recovered within a couple of atomic planes far from the interface, so that the band alignments are well defined.