We describe a novel assay format for the Gal4-based yeast two-hybrid-system, in which the readout from three different reporter genes is measured sequentially in a single microplate. Activation of the URA3, MEL1, and lacZ reporters in response to a protein-protein interaction is monitored by measuring sequentially: (i) growth in medium lacking uracil, (ii) alpha-galactosidase activity, and (iii) beta-galactosidase. The data thus generated permit elimination of many false positive signals and provide a preliminary measurement of reporter activation-strength that may be confirmed by further analysis. The assay procedure is inexpensive and requires few liquid-handling steps. It is appropriate for automated high-throughput interaction mating assays, validation of putative interactor strains and hybrid-protein self-activator tests.