Abstract: The present invention provides methods to measure phosphorylation and transphosphorylation by histidine kinases proteins and assays to detect modulators of histidine kinase activity. The method of the invention is a robust, sensitive assay of simple design that is easily automated and an assay design that can be easily modified to allow different histidine kinase and response regulator targets to be tested without significant modification to the design. The present invention relates to scintillation proximity assays (SP) useful to screen compounds that modulate histidine kinase function.