The isotropic and anisotropicexchange energies in phosphide and arsenide compoundswith helical orferromagneticstructureshave been studiedby meansofan anisotropic twc-band model. The anisotropic term is separated into a symmetric and an antisymmetric contribution. It appears that, incrystals with non-vanishingspin-orbit interaction, acircular isotropic helm minimizes the antisymmetric contribution (AEI) to the exchange energy. The helix is stable if the AEI compensates for the increase in the symmetric anisotropic energy that the deviation from a non-collinear spin arrangement causes. The band model predicts the correct ground state. with the exception of MnP. where the helical (ground state) and the ferromagnetic (high-temperature phase) configurations correspond to the same energy. Theresuluconfirmthatthe~~land theshapeoftheFermisurface arecrucial for thestability of a helical magnetic structure.