One hundred and twenty four water-insoluble drugs were included in a study for their action on diamine oxidase (DAO) after solubilization with 61 detergents. 16 detergents were themselves not water-soluble and were not further investigated. A further 3 detergents affected the extraction procedure and 7 of the remaining 42 detergents themselves inhibited the activity of canine intestinal DAO in vitro. Only 5 detergents fulfilled all prerequisites for our DAO assay, including the solubilization of 76 water-insoluble drugs. Each of these 5 detergents had an individual range of suitability in our test system. 3/76 drugs inhibited DAO in concentrations up to 10(-3) M. This result is in contrast to our study with water-soluble substances, where 16% were DAO inhibitors. Since detergents can block the enzyme which is responsible for histamine catabolism, some of the observed adverse reactions to drugs could arise because of the presence of such detergents in the formulation.