Positivegiant magnetoresistance (GMR), &Dgr;&rgr;/&rgr;, is reported here in polycrystalline antiferromagnetic materials RE2Ni3Si5(RE=Tb, Sm, Nd); &Dgr;&rgr;/&rgr;, at 4.4 K and in a field of 45 kOe, is 85%, 75%, and 58%, respectively. Positive GMR of such large magnitude has not been reported earlier in magnetically orderedpolycrystallinecompounds. The observed GMR is not correlated to the RE‐moments. It is, however, associated with the magnetic ordering of the lattice as its magnitude is significantly reduced in the paramagnetic state. Surprisingly, MR in Y2Ni3Si5, a non‐magnetic rare earth analogue, is also relatively large (16% at 4.4 K; 45 kOe) and is even slightly higher than that of antiferromagnetically ordered Gd2Ni3Si5(12% at 4.4 K; 45 kOe). The layered structure of the materials is suggested to be responsible for the observed GMR. ©1996 American Institute of Physics.