The present shortage of fast reactor test space, particularly for test regions > 2cm in diameter, led to a series of tests to develop an adequate spectrum-hardening filter so that a suitable fast-neutron flux environment could he obtained in a large thermal test reactor. A boron filter was the best of a number of filter materials tested, and verification measurements were made in the Engineering Test Reactor II (ETR) Critical Experiment. After several design modifications, fast-neutron flux spectra typical of the Experimental Breeder Reactor II (EBR-II) were obtained at levels ∼½ of those obtainable in the peak region of the EBR-II but with a test hole diameter of 3.5 cm. Softer neutron spectra, typical of some of the proposed fast breeder designs, can be obtained in a filtered ETR experiment with fissile fission rates greater than those in the EBR-II.