  A very large negative magnetoresistance (LNMR) is observed in the insulating
regime of the antiferromagnet BaMn$_2$Bi$_2$ when a magnetic field is applied
perpendicular to the direction of the sublattice magnetization. High
perpendicular magnetic field eventually suppresses the insulating behavior and
allows BaMn$_2$Bi$_2$ to re-enter a metallic state. This effect is seemingly
unrelated to any field induced magnetic phase transition, as measurements of
magnetic susceptibility and specific heat did not find any anomaly as a
function of magnetic fields at temperatures above $2\,\mathrm{K}$. The LNMR
appears in both current-in-plane and current-out-of-plane settings, and Hall
effects suggest that its origin lies in an extreme sensitivity of conduction
processes of holelike carriers to the infinitesimal field-induced canting of
the sublattice magnetization. The LNMR-induced metallic state may thus be
associated with the breaking of the antiferromagnetic parity-time symmetry by
perpendicular magnetic fields and/or the intricate multi-orbital electronic
structure of BaMn$_2$Bi$_2$.
