AbstractThe presence of peroxisomes and their enzymatic content were investigated and compared in healthy and neoplastic human breast epithelial cells using cytochemical studies at the ultrastructural level as well as Western blot and biochemical analyses. Ultrastructural cytochemistry revealed the presence of these organelles in both normal and neoplastic breast tissues. Their mean diameter was 0.27 ± 0.11 μm. No significant difference was noted between numbers of peroxisomes in normal and neoplastic breast epithelia. Catalase,D‐amino acid oxidase, and urate oxidase were found to be expressed in mammary carcinoma and in surrounding non‐malignant tissue when the postnuclear supernatant fractions prepared from homogenates were assessed by Western blot techniques. Their specific activities and that of fatty acyl CoA oxidase as determined spectrophotometrically were found to be diminished in the tumour when compared with the control tissue. On the other hand, no significant difference was found in the specific activity of theL‐a‐hydroxy acid oxidase of normal and neoplastic human breast tissues. Investigations of the relationship between peroxisomal enzymes and tumour grade revealed that catalase, urate oxidase, and fatty acyl CoA oxidase activites in breast neoplastic tissues belonging to grade III were significantly lower than in the adjacent norma