  To reconcile the measurements of 3He/H in Galactic HII regions with high
values of 3He in a couple of planetary nebulae, we propose that thermohaline
mixing is inhibited by a fossil magnetic field in red giant stars that are
descendants of Ap stars. We examine the effect of a magnetic field on the
salt-finger instability, using a local analysis. We obtain a threshold for the
magnetic field of 10^4 - 10^5 Gauss, above which it inhibits thermohaline
mixing in red giant stars located at or above the bump. Fields of that order
are expected in the descendants of the Ap stars, taking into account the
contraction of their core. We conclude that in a large fraction of the
descendants of Ap stars thermohaline mixing does not occur. As a consequence
these objects must produce 3He as predicted by the standard theory of stellar
evolution and as observed in the planetary nebulae NGC3242 and J320. The
relative number of such stars with respect to non-magnetic objects that undergo
thermohaline mixing is consistent with the statistical constraint coming from
observations of the carbon isotopic ratio in red giant stars. It also satisfies
the Galactic requirements for the evolution of the 3He abundance.
