Central reactivity worth measurements have been made in a fast reactor spectrum with samples of natural boron, boron-10, europium oxide, and tantalum. Various sized samples were used to investigate self-shielding effects in a fast reactor test region in Assembly 17 of the Epithermal Critical Experiments Laboratory. In addition to single cylinders, clusters of tantalum pins simulating a control rod segment were also used. Compared to an infinitely dilute sample, the most massive tantalum sample showed a reduction of 49 percent in reactivity per unit mass. For comparison with the tantalum measurements, extensive calculations using first-order perturbation theory, exact perturbation theory, and eigenvalue differences show good agreement within appropriate ranges—first-order perturbation for small perturbations, eigenvalue differences for large perturbations, and exact perturbation throughout the range. For europium, first-order perturbation calculations are in excellent agreement with the measurements, while for boron and B, the calculations predict a somewhat greater worth than was measured. By using the calculations to extrapolate the measurements, the following infinitely dilute specific reactivity values are obtained: boron, -55.8 m/g; 10B, −293.8 mg; europium, −20.6 mg; and tantalum, −5.83 m/g.