Relationships based on the microdosimetric-kinetic model are presented that calculate the average number of lethal lesions, and the associated cell survival, produced in mammalian cells by exposure to protracted continuous irradiation by temporary and permanent implantation of radioactive sources. The influence of cell parameters of linear-quadratic survival, repair function and proliferation rate, as well as the influence of dose rate, isotopic decay rate and linear energy transfer (LET) quality on cell killing are displayed and discussed. An expression for biologic effective dose (BED) is presented that facilitates comparison of the effects of protracted low-dose-rate irradiation and with a course of multiple instantaneously administered radiation treatments (fractions).