Device for testing the potency of a biological control reagent, as in antibiotic susceptibility testing, especially the determination of minimum inhibitory concentration, comprising a strip of a plurality of interconnected plastic cups having a capacity of about 0.3 ml. each. All but two of the cups contain a range of amounts of the control reagent in an essentially dry state which will provide a predetermined range of concentrations when the reagent is redispersed. Contamination of the dry reagent in the cups is prevented by an adherent plastic film or aluminum foil across the openings of the cups. In use, the adherent film or foil is removed, the dry reagent is redispersed to form the required range of concentrations, and all of the cups except one are inoculated with the biological material against which the potency of the control reagent is to be tested. The last cup contains the dispersion medium but not the control reagent nor inoculum, and provides a sterility control. The second last cup contains the dispersion medium and the inoculum, and serves as a growth control. The strip of cups is then resealed, incubated, and the test results are read.