Kneisel, Ciovati, Myneni and co-workers at TJNAF have recently fabricated two superconducting cavities from the center of a large grain Nb billet manufactured by CBMM. Both cavities had excellent properties with one attaining an accelerating gradient of 45 MV/m (2 K) after a 48 h and 120 °C bake [P. Bauer et al., An investigation of the properties of BCP niobium for superconducting RF cavities, in: K.-J. Kim, C., Eyberger (Eds.), Proceedings of the Pushing the Limits of RF Superconductivity workshop, Argonne National Laboratory Report ANL-05/10, March 2005, pp. 84–93]. An investigation is underway to use magneto-optical (MO) imaging to observe the flux penetration behavior of a sheet sliced from this billet. The large grain size (some larger than 50 mm) allowed us to isolate multiple bi-crystals and tri-crystals. In the first stage of the present study we have taken the as-received sheet (RRR ∼280), which has been etched to reveal the grain structure. By magneto-optical examination we observed preferential flux penetration at some grain boundaries of a bi-crystal where the grain boundary was almost perpendicular to the sample surface and there was <1 μm surface step across the boundary. At other grain boundaries, with large steps or where the grain boundaries were not normal to the surface, we observed no preferential flux penetration. Preliminary transport measurements on a bi-crystal showed greater normal state resistance and lower superconducting critical current at the grain boundary.