The compound, Tb 5 Si 3 , crystallizing in Mn 5 Si 3 -type hexagonal structure, was recently reported by us to exhibit a sudden and huge enhancement in electrical resistivity (ρ) at a critical magnetic field (H cr ) in the magnetically ordered state (< 70 K) tracking isothermal magnetization (M) behavior. We have investigated the influence of external pressure (≤15 kbar) and negative chemical pressure induced by Ge substitution for Si on M and ρ as a function of temperature (5-300 K) and magnetic field (<120 kOe), with the primary aim of understanding the field-induced anomalies. Focusing on isothermal M and magnetoresistance (MR) at two temperatures, 5 and 20 K, we find that this ρ anomaly persists under external as well as negative chemical pressures, however, with a change in the H cr . The pressure-derivative of H cr is negative and this trend and the MR behavior at the H cr are comparable to that observed in some Laves phase itinerant magnetic systems. On the basis of this observation, we speculate that the magnetic fluctuations induced at this critical field could be responsible for the MR anomalies. INTRODUCTION The process of metamagnetism [1], in which a low magnetic moment state of a magnetic material gets transformed to a high magnetic moment state with the application of an external magnetic field (H ext ), continues to attract attention in condensed matter physics. Some phenomena like 'giant magnetoresistance' and the 'giant magnetocaloric effect', which are of great current interest, are closely associated with this process. In particular, for the case of rareearth (R) intermetallics, such a field-induced change in magnetism has been widely reported in the literature. In these local-moment systems, the electrical resistivity (ρ) at the metamagnetic transition usually decreases resulting in negative magnetoresistance, MR [defined as {ρ(H)-ρ(0)}/ρ(0)], due to a reduction in magnetic scattering by the application of magnetic field (H). The reader may see, for instance, Ref. 2 for the behavior of the MR in Gd 5 Si 2 Ge 2 and in DySb following the metamagnetic transition. In contrast to this general behavior, recently we observed [3] an enhanced positive MR at the field-induced magnetic transition in the case of a localmoment system, Tb 5 Si 3 . We considered it important to obtain more experimental results on this compound to understand the origin of such a MR anomaly.At this juncture, it is to be noted that the itinerant systems [4] exhibiting the field-induced magnetic anomalies gained considerable theoretical and experimental interest. In particular, the above mentioned MR anomaly was in fact observed in the cubic Laves phase family, RCo 2 . These binary compounds have been subjected to intense experimental investigations over a period of two decades [5][6][7][8][9]. In these systems, the existence of field-induced transitions and MR anomalies is known to originate from the existence of a peak in the Co 3d density of states in the vicinity of Fermi level. Yamada [7] theoretically predicted that the ...