As reported in previous investigations, erythrocytes are the elements of peripheral blood most affected by free radical activity in the pathogenesis of cancer. In these studies, the level of sulphydrilic groups and reduced glutathione were assayed in the erythrocytes and plasma, while their successful scavenger activity against cell membrane oxidation and peroxidation has already been established. In subjects with cancer, the levels of -SH groups (p < 0.002) and reduced glutathione in both plasma and erythrocytes (p < 0.0001) were shown be a statistically significantly decreased compared to healthy controls. These differences were related to the defence of the hematic tissue against free radical activity. A similar pattern has also been reported when studying vitamin A and E content in the peripheral blood of cancer patients. The role of oxido-reduction phenomena in this disease is discussed, as well as the importance of reducing the oxido-peroxidation involvement of tissues and cell elements. The study of the GSH/GSSG ratio in order to determine the stage of the disease would be useful and might represent a systemic marker for cancerous lesions.