a method for the assay of n samples each containing a compound to be tested , comprises providing n reaction vessels each containing a population of carrier beads and other reagents for performing the assay , where n is at least 2 e . g . 80 - 4000 . each population of carrier beads is distinguishable from every other population . after adding the samples to the reaction vessels and performing the assays , the contents of all the reaction vessels are mixed and subjected to analysis by flow cytometry . by means of flow cytometry , each carrier bead is rapidly analysed to identify its population and also to determine the presence or concentration or biological activity of the compound to be tested .