We have performed measurements of the temperature dependence of the magnetoresistance up to 9 T in bulk single crystals of UPt3 with the magnetic field along the b-axis, the easy magnetization axis. We have confirmed previous results for transverse magnetoresistance with the current along the c-axis, and report measurements of the longitudinal magnetoresistance with the current along the b-axis. The presence of a linear term in both cases indicates broken orientational symmetry associated with magnetic order. With the current along the c-axis the linear term appears near 5 K, increasing rapidly with decreasing temperature. For current along the b-axis the linear contribution is negative.