Self-consistent, spin-polarized energy-band calculations have been performed for MnSb, for both ferromagnetic and antiferromagnetic spin alignments. For ferromagnetic MnSb we find a 3d5.5 configuration on the Mn atom, with a magnetic moment of 3.3μB on the Mn sites, and –0.06μB on the Sb sites. The charge transfer of Mn to Sb is very small. A detailed analysis is made of the strong covalent Mn-Mn and Mn-Sb interactions. The coupling between the moments is described in terms of covalent interactions of unoccupied states of the Sb 5p band with the Mn 3d bands. These interactions stabilize the ferromagnetic alignment. The calculated spin density, which agrees very well with experimental data, can be easily understood in terms of a model of covalent bonding. We compare the calculated Fermi surface, total density of states, and joint density of states. with results of transport measurements, x-ray photoelectron spectroscopy, and optical measurements, respectively.