An ultrasensitive assay for measuring DNA base damage is described that couples immunochemical recognition with capillary electrophoresis and laser-induced fluorescence detection. The method provides a detection limit of 3 x 10(-21) moles, an improvement of four to five orders of magnitude over current methods. Induction and repair of thymine glycols were studied in irradiated A549 cells (a human lung carcinoma cell line). Exposure of these cells to a low dose of radiation (0.25 Gray) 4 hours before a clinically relevant dose (2 Gray) enhanced removal of thymine glycols after the higher dose. These data provide evidence for an inducible repair response for radiation-induced damage to DNA bases.