INTRODUCTION Breast cancer is a disease of worldwide importance since it is considered the 5th leading cause of cancer deaths. Triple-negative breast cancer (TNBC) is a molecular subtype that presents resistance to conventional radiotherapy, demanding high doses of ionizing radiation (IR) for a prolonged period of treatment. On the other hand, low-level light irradiation (LLLI) has been studied to sensitize cells before IR exposure. However, the literature is poor regarding the association of both techniques in TNBC cells. OBJECTIVES Thus, we aimed to assess the effect of LLLI before IR exposure on two TNBC cell lineages. MATERIALS AND METHODS MDA-MB-231 (human TNBC) and 4T1 (murine TNBC) were cultivated, seeded at a density of 2.5 x 10 5 cells/cm??, and maintained in an incubator (37??C, 5% of CO2) overnight. LLLI was performed with a red LED (??= 660 ?? 11 nm, 38.2 mW/cm??) delivering energies of 1.2 J and 6.0 J. One-h after LLLI, the cells were submitted to both 2.5 and 5.0 Gy doses from a 60 CO source. After 24-h, mitochondrial activity (MA) was quantified by MTT assay with n= 9/group. DISCUSSION AND RESULTS Our data showed that 4T1 cells exposed to LLLI at 1.2 J exhibited higher MA than cells exposed to IR2.5. In contrast, cells exposed to 6 J of LLLI showed lower MA than IR5. Concerning MDA-MB231 cells, no statistically significant differences were noticed among groups regardless of IR and LLLI doses. CONCLUSION These findings indicate that LLLI before IR could sensitize only murine breast cancer. Besides, an appropriate combination of IR and LLLI doses seems to play a role to kill TNBC cells.