A number of changes in peptide components could be demonstrated in sera of patients with end stage malignant diseases. Total and free alfa amino-N content of such sera was shown to be significantly higher than those in sera of patients with no sign of malignancy. In addition, a significant increase was found in one of the serum fraction obtained by Sephadex G-25 chromatography. This increase was shown to be due to an increase in the amount of one of the isotachophoretically separated serum components of anionic character as well as to the appearance in sera of tumour bearing patients of two additional isotachophoretic components never detected in non tumour bearing patients. Based on their chromatographic behavior as well as on observations made in earlier experiments, the peptide nature of the two isotachophoretic serum components seemingly characteristic of sera of tumour bearing patients is highly probable.