By combining existing linear-quadratic equations relating to decaying-source therapy with an assumed tumour repopulation factor, it has been possible to devise a method for the radiobiological assessment of permanent implants. For calculation purposes there is a time after which an implant can no longer be considered effective in sterilizing tumour cells. This "effective" treatment time for a permanent implant can be approximately defined in terms of the radionuclide decay constant, the potential doubling time, the initial dose-rate and the value of alpha in the tumour alpha/beta ratio. The analytical technique has been applied to a specific intercomparison of commonly encountered implants using 125I and 198Au, and suggests that, even in the most favourable cases, the former radionuclide offers few radiobiological advantages. Although not specifically discussed here, the method can also be applied to the assessment of various forms of biologically targeted radiotherapy.