The activity of cysteine peptidases (cathepsins B and L) was estimated in homogenates of tissues sampled during surgery from 60 patients operated due to colorectal tumors. The results were compared to those obtained using tissues in which histopathology disclosed no tumorous cells, obtained from 20 patients of the same group, treated as a control. Activity of the enzymes was inhibited using cysteine peptidase inhibitors isolated from chicken egg proteins. Application of the inhibitors was found to inhibit activity of the enzymes which play a key role in tumor development. It is suggested that in future the inhibitors may provide a component of new generation drugs in the so-called inhibitor therapy.