The detection of alkaline phosphatase (ALP) activity is used as a legal test to determine whether milk has been adequately pasteurized or recontaminated with raw milk. However, a wide variety of microorganisms produce both heat labile and heat stable ALPs which cannot be differentiated from the milk ALP by current enzymatic methods. Monoclonal antibodies specific of the bovine milk ALP were obtained in mice from a raw bovine milk ALP preparation. Coated in microtitre plates, these antibodies specifically capture the bovine milk ALP from dairy products. After washing, the enzymatic activity of the captured ALP is revealed by adding p-nitrophenyl-phosphate as a substrate. This simple immunoassay does not react with ALPs of intestinal or bacterial origin and, once optimized, was found to be the first immunoassay suitable to detect raw milk in boiled milk down to a 0·02% dilution. Moreover, in contrast with competitive indirect ELISA formats, the capture immunoassay does not require purified ALP.