The aim of the proposed study is to assess the clinical effect of photodynamic therapy (PDT) on dentin with carious lesions in primary teeth. Patients with primary molars exhibiting deep carious lesions on the occlusal surface indicated for restorative treatment will be randomly allocated to three groups: Group 1 - traditional caries removal with a low-speed drill; Group 2 - PDT + PapaMblue (carious tissue removal agent) modified with methylene blue; and Group 3 - PDT with methylene blue 0.05%. PDT will be performed with low-level laser for the treatment of the carious tissue. Dentin samples will be removed before and after PDT for microbiological analysis. The microbiological samples will be cultured in Brucella blood agar, Mitis Salibarius-bacitracin agar and Rogosa SL agar. The teeth will then be restored using high-viscosity glass ionomer cement, with clinical and radiographic follow up at six, 12 and 24 months. The data will be submitted to descriptive statistics.