The present invention relates to fluorescence polarization assays for determining the HDC modulating activity of a candidate compound including providing a reaction mixture comprising a HDC, histidine, a fluorescently labeled histamine probe, a candidate compound and an anti histamine antibody having selectivity for histamine at least 10 fold greater than histidine: incubating the reaction mixture; and determining whether inhibition of HDC has occurred in the presence of the test compound, wherein an increase in fluorescence signal is an indication that the test compound inhibits the activity of the HDC.