Leukocyte peroxidase deficiency has been demonstrated in a confirmed case of neuronal ceroid-lipofuscinosis with guaiacol, o-dianisidine, and p-phenylenediamine used as hydrogen donors in the peroxidase assay system. Nitroblue tetrazolium (NBT) reduction values in the leukocytes of the patient were also found to be significantly higher than those of normal controls, indicating the impaired hydrogen peroxide catabolism. When the patient was given a daily dose of vitamin E (400 I.U.), vitamin C (1 gm), and methionine (1 gm.) along with a weekly intramuscular injection of vitamin B12, the leukocyte peroxidase values of the patient returned to normal levels in about 7 weeks. NBT reductions values also decreased to normal levels. The regenerated enzyme in the patient's leukocytes was shown to have similar chromatographic and electrophoretic properties as the leukocyte peroxidase of normal controls. After about 28 weeks of therapy, the peroxidase levels in the leukocytes of the patient returned to original low levels, with concomitant increase in the NBT reduction values. The effect of vitamin therapy on normal control subjects was, at least in some cases, an increase of leukocyte peroxidase. A significant increase in the peroxidase levels of the patient's leukocytes during vitamin therapy remains unexplained, and the possibility of peroxidase deficiency being a secondary manifestation rather than the primary defect in Batten's disease cannot be ruled out.