A new microplate scintillation and luminescence counter (TopCount™ HTS, Packard) has been developed for counting both 96- and 384-well microplates. The first assay methods tested in the 384-well plate were the scintillation proximity assay and the luciferase reporter gene assay. The binding curve and luciferase induction were identical in either plate format. The signal-to-noise ratio (maximum binding divided by nonspecific binding) in the 384-well plate is equal to or greater than those measured in the 96-well format resulting in a comparable assay window. Reagent pipetting for SPA, including the scintillating beads, was performed with the MultiPROBETm 104 Automated Liquid Handling System (Packard, Meridan, CT). This instrument has both the positioning and pipetting accuracy to support the 384-well microplate; therefore, making complete automation practical. With little or no optimization, both the SPA and the luciferase reporter gene assay could be miniaturized to 50
,Il
volumes or less.
