The effect of very low single X-ray doses (0.05-4 Gy) was investigated in a human lung epithelial cell line (L132). Cell survival measurements were made using a Dynamic Microscopic Imaging Processing Scanner (DMIPS), which allowed single cells to be located accurately, their positions recorded and these positions revisited after an appropriate incubation period at 37 degrees C; surviving cells were identified by their ability to produce a colony > or = 50 cells. The survival data at doses > or = 2 Gy were well-fitted by a linear-quadratic (LQ) model. For doses < 1 Gy, increased X-ray effectiveness was observed with cell survival below the prediction from the fit of the LQ model to the higher dose data, extrapolated into the low dose region. This is the first evidence for the existence of a hypersensitive survival response to very low doses in normal human cells. The transition between the low dose hypersensitive region and greater resistance at higher doses, could result from induced radioresistance which requires a threshold of radiation-induced damage before being triggered.