  We investigate the magnetic field behaviour of an antiferromagnetic
Heisenberg spin-1 chain with the most general single-ion anisotropy. We discuss
the regime in which the magnetic field is below the transition value. The
splitting of the Haldane triplet is obtained as a function of a field applied
in an arbitrary orientation by means of a Lancz\H os exact diagonalization of
chains of up to 16 spins. Our results are nicely summarized in terms of a
first-order perturbation theory. We explain various level crossings that occur
by the existence of discrete symmetries. A discussion is given of the electron
spin resonance and neutron scattering experiments on the compound
Ni(C$_2$H$_8$N$_2$)$_2$NO$_2$ClO$_4$ (NENP).
