An ion bombardment experiment was designed to investigate the minimum critical radius, r^cc*, for the ferritic alloy Fe-10Cr. Specimens were implanted with 300 atomic parts per million (appm) helium, annealed, and then irradiated to 30 displacements per atom (dpa) at 850 K with 4 Me V Fe⁺⁺ ions. The specimens contained a bimodal cavity distribution consisting of a population of larger cavities (average radius 7.6 nm) and a population of smaller cavities (average radius 1.2 nm). The upper cut-off of the cavity radii for the smaller cavities, 2.5 nm, is interpreted as r^cc*. Theoretical calculations of r^cc* for physically allowable combinations of bias, surface energy, vacancy migration energy, and vacancy formation energy and entropy were performed using the measured minimum critical radius and microstructural data. Thus, an estimation of these fundamental parameters was made for this alloy. The results suggest that a bias of ∼0.2 is reasonable. An assessment of the possible values of the other fundamental parameters is given. In addition, the low irradiation-induced dislocation density (1 × 10¹³/m²), which results in the ratio of the dislocation and cavity sink strengths, Q, being much less than unity, may be partially responsible for the low cavity growth rate in this alloy.