To investigate radiosensitization by halogenated pyrimidines in human cells, SW-1573 lung tumor cells were grown in absence or presence of 4 mu M of iododeoxyuridine (IdUrd). Cell survival after irradiation with gamma-rays from a Cs-137 source with single doses up to 8 Gy, was determined by clonogenic assay, Radiosensitization by IdUrd was observed in both exponentially growing and plateau phase cells. Linear-quadratic analyses of the radiation survival curves demonstrated that radiosensitization was mainly expressed by an increase of the value of alpha (determining the initial slope of the survival curve) which can be interpreted as an enhancement of fixation of potentially lethal damage (PLD). The value of beta (determining the curvature of the survival curve), which might be attributed to enhanced interaction or to fixation of sublethal damage (SLD), was not influenced. In view of clinical applications of IdUrd it is of interest that radiosensitization has its main effect in the low dose region of the cell survival curve, described by the linear term alpha D.