Abstract: A dilution and mixing cartridge is provided that allows single (or multiple) dilutions of a sample with a diluent in a disposable cartridge in which a measurement, such as optical density, is made. Addition of sample to the device automatically measures the sample, and addition of diluent automatically causes a fixed ratio of sample and diluent to enter a receiving chamber, in which mixing and measurement can take place. A number of preferred embodiments are provided in which the diluent application site is located at a vertical height below the sample application site, flow directing chambers are provided to control flow of desired volumes of sample or diluted sample at the appropriate locations in the device and to direct excess sample and diluent to waste locations, or control mechanism are provided at flow directing chambers and other internal locations to verify correct operation of the device.