A chemiluminescent method for determining xanthine oxidase (XOD) activity was developed and applied to the assay of milk enzyme activity using a photomultiplier luminometer. Various kinds of milk and cream samples were analysed for XOD content. In pasteurized milk, XOD activity depended on the fat content and in UHT milk it disappeared owing to the heat treatment. Milk sample preparation was very simple, requiring only homogenization at 40°C followed by a 1[ratio ]10 dilution with UHT (‘XOD-free’) milk. The assay was carried out at 25°C. The response obtained from XOD standard solutions in milk was linear from 0·1 to 500 enzyme units (U) l−1, but for the actual milk samples values ranged only from 1 to 135 U l−1. The detection limit at 2 SD was 0·1 U l−1 in milk, while in buffer it was 100 times lower. The intra-assay and interassay CV for XOD activity in milk were 6–12%.