The process of tumor growth is a phenomenon which, if understood better, could greatly improve the diagnostic and treatment of patients. In this context, the use of polarimetric imaging can offer more information than classical imaging methods. Here we use a new kind of calibration-free spectro-polarimeter which can be helpful for optical biopsy. Fifty different mice have been studied in full Mueller polarimetry with this device. Some were injected with very pigmented melanoma cells, others with non-pigmented breast cancer cells. Variations in depolarization were measured throughout this study: melanomas were accompanied with intense drops in depolarization, while mice injected with breast cancer cells showed a more diffuse decrease in depolarization. The study confirmed the potential of polarimetric imaging as an optical biopsy tool.