  A recent theory of magnetoresistance in positionally disordered organic
semiconductors is extended to include exchange and dipolar couplings between
polarons. Analytic results are discovered when the hyperfine, exchange, and
dipolar interactions have little time to operate between hopping events. We
find an angle-of-field dependence of the magnetoresistance that agrees with
previous experiments and numerical simulations. In addition we report new
magnetoresistive behavior that critically depends upon the amount of anisotropy
in the dipolar interaction.
