Bodily fluid is analyzed for the presence of drugs of a selected panel of drugs in a simultaneous assay in which sample of the fluid is incubated with additional amounts of all drugs of the panel, antibodies specific to each of the drugs of the panel, and microparticles, the microparticles being divided into subsets, one subset for each drug in the panel and each subset distinguishable from the others. The incubation is performed in a liquid medium in which competitive binding occurs, the drugs in the sample competing with those added to the assay medium for binding to the antibodies. In one procedure, the added drugs are pre-coupled to the microparticles while the antibodies are not, and the incubation is followed by further incubating the microparticles with labeled ligands that have affinity for the antibodies. In an alternative procedure, the added drugs are not coupled to the microparticles but are pre-labeled, while the antibodies are pre-coupled to the microparticles, and the assay proceeds without further incubation. In both alternatives, the microparticles are ultimately recovered from the assay medium and from any unbound species, and the recovered microparticles are analyzed by flow cytometry to obtain indications of the presence of the various drugs in the sample in an inverse manner by detection of the label, each drug differentiable from the others by the distinguishing features of the microparticles.