A reagent cartridge for a assay device comprises a frame and at least one reagent container. A reagent cartridge frame comprises a plurality of sidewalls, one of the sidewalls includes at least one detent to engage with a flange on a reagent container, and another sidewall includes at least one notch to engage with a rib on the reagent container. The reagent cartridge frame permits various numbers of reagent containers to be placed in the frame at any position and in any order depending on the assay being performed. The reagent cartridge may also be used with as few as one reagent container.