Studies of amniotic fluid obtained at various times throughout the last trimester of pregnancy from patients with carcinoma of the cervix indicate a complete deficiency of peroxidase activity. Dialysis partially restores the enzyme activity. Cervical epithelial tumor cells apparently release a dialyzable, low molecular weight inhibitory substance directed specifically toward peroxidase. If this proves to be true in future studies, the peroxidase assay could be a sensitive means of differentiating between carcinoma and dysplasia. A peroxidelike substance was also present in the amniotic fluids obtained from patients with carcinoma. Other investigators have demonstrated large amounts of peroxide in malignant tumors. These elevated peroxide levels might well be directly related to deficient peroxidase activity rather than being a result of other abnormal enzyme levels which have been regarded as being principally involved in the metabolism of this highly toxic molecule.