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Diamine oxidase (DAO) oxidatively deaminates histamine and other diamines. Due to the lack of antibodies for human DAO, many findings on this enzyme had not been confirmed in man. Therefore, we produced a series of monoclonal antibodies by immunizing mice with human DAO protein fragments expressed in vitro. Five different monoclonal antibodies specific for human DAO were obtained that do not recognize any other human protein and can detect DAO with 100-fold greater sensitivity than the most sensitive enzymatic assays currently available. Using these antibodies allowed confirming the expression and cellular localization of DAO in various human tissues such as kidney, intestine and placenta where the presence of the enzyme had previously been deduced from activity measurement and DAO mRNA analysis. Due to the high sensitivity of the novel monoclonal antibodies, DAO was also detected at sites that previously evaded unequivocal proof of DAO enzymatic activity such as the urine. On the other hand, with these antibodies it was possible to show that DAO is normally not present in human liver and blood serum. The new monoclonal antibodies not only allow a comprehensive quantitative evaluation of the expression of DAO at the cellular level in man but will also facilitate sensitive analyses of disease-associated alterations of this enzyme. [ABSTRACT FROM AUTHOR]