The Nd monopnictides NdP, NdAs and NdSb are simple cubic type I antiferromagnets in which the crystal-field splitting is larger than the exchange energy. The magnetic properties are calculated by means of a mean-field theory including crystal-field and magnetoelastic effects. The calculations are based on energy levels determined by inelastic neutron diffraction. Exchange constants are derived by comparing the theoretical paramagnetic susceptibility with experiments over a large temperature range. The temperature dependence of the sublattice magnetization deviates significantly from the usual molecular field curve (for J=9/2) which neglects crystal-field effects. Theoretical high-field magnetization curves are presented. The observed tetragonal distortion below the Neel temperature TN is explained by the theory. The effect of biquadratic exchange interaction is investigated.