Enhanced collagenolytic activity was observed in carious dentin lesions, suggesting the presence of MMPs and other proteolytic enzymes in carious lesons. Here, we have studied the involvement of cysteine proteinases in carious dentin lesions. It was selected 39 patients from Universidade de Mogi das Cruzes Dental Clinic. The amount of cathepsin cysteine proteinases in carious tissues were monitored by using the fluorogenic substrate Z-F-R-MCA in the presence of the specific cysteine proteinases inhibitor E-64 and by immunohistochemistry technics. Our data show that cysteine proteinase level increase in proliferative carious lesions with pulp exposure. On the other hand, superficial carious lesion cathepsin activity is the same as with healthy dentin samples. Immunoperoxidase staining for cathepsin B in dentin shows that this enzyme was detected in odontoblast cells and showed peritubular disposition around dentinal tubules. Therefore, our data strongly suggest that cysteine cathepsins come from dentin matrix, or pulpal dentinal fluid, or both. Also, cysteine proteinase level in healthy dentin decreases with age. The decrease of cysteine proteinase in dentin matrix may also be, at least partially, the reason for the lower levels of activity observed in carious dentin. Alternatively, pulpal dentinal fluid, containing cysteine proteinases, is much more accessible to caries lesion in young teeth with wide open dentinal tubules. Indeed, during primary dentinogenesis, caries progression is rapid, but it slows as soon as secondary dentinogenesis is involved, and may discontinue almost completely when tertiary (reparative) dentinogenesis is active. Our data agree with these observations and show that cathepsin cysteine proteinases are related in development and progression of human carious dentin lesions. (Supported by CAPES, CNPq and FAPESP)