An apparatus and method for real-time measurement of a cellular response of a test compound or series of test compounds (303) on a flowing suspension of cells (349), in which a homogeneous suspension of each member of a series of cell types (349) is combined with a concentration of a test compound (303), directed through a detection zone (355), and a cellular response of the living cells is measured in real time as the cells in the test mixture are flowing through the detection zone (355). The apparatus may be used in automated screening of libraries of compounds, and is capable of real-time variation of concentrations of test and standard compounds and generation of dose/response profiles within a short timespan.