In traditional ABC and SP methods, immunohistochemical staining can easily be interfered by endogenous peroxidase or biotins, therefore an inactivation step is needed. Endogenous peroxidase can be inactivated by perhydrol or avidin, for 10 min of 3% perhydrol or 10-30 min of 0.3% perhydrol. It's better to dissolve perhydrol in methanol rather than in ddH2O or PBS buffer for a better antigen protection and fixation. A too long incubation time might lead to section shedding. Inactivation buffer should be stored at 4°C in darkness.