Experimental programs to investigate the physics characteristics of heterogeneous liquid-metal fast breeder reactor cores have been conducted in the zero-power plutonium reactor critical facility over a period of ∼ 5 yr. Previous experiments on conventional homogeneous cores provided appropriate benchmark data against which to judge the heterogeneous core results. For a heterogeneous reactor of the Clinch River Breeder Reactor size, both the physics parameters and the ability to predict them by common design methods differ substantially from an equivalent conventional design. Data errors and methods approximations have a greater effect in the analysis of heterogeneous cores, particularly with respect to such spatially varying parameters as power distributions and control rod worths. Preliminary results from recent experiments on a 700-MW(electric)-sized heterogeneous assembly are presented. As expected, predictions of physics parameters in general are worse than for conventional cores. Eigenvalue spectra and cross-section sensitivity have been used to characterize the spatial sensitivity of the cores.