  We develop a theory of magnetoresistance based on variable-range hopping. An
exponentially large, low-field and necessarily positive magnetoresistance
effect is predicted in the presence of Hubbard interaction and spin-dynamics
under certain conditions. The theory was developed with the recently discovered
organic magnetoresistance in mind. To account for the experimental observation
that the organic magnetoresistance effect can also be negative, we tentatively
amend the theory with a mechanism of bipolaron formation.
