The extensive use of commercial kits in molecular biology and biochemistry has prompted us to design a series of practical sessions to help students become familiar with the uses and limitations of pre-packaged assay systems. To facilitate an understanding of these assay systems and to promote reflection on their appropriate use, students manufacture their own kit for one of four enzyme-linked metabolite assays. To do this they must investigate the role of each of the components in the assay, optimize the conditions where possible, check for cross reactivity, and then work this assay up for a few related "real" samples. Students make up all the buffers, the standard solution, instructions, and even the packaging. The kit is checked for accuracy by the producers, "marketed" to other students, and evaluated by their peers. By going through this process, students learn the benefits and pitfalls of commercial kits as well as reinforcing the basic principles of metabolite measurement and gaining experience with assay design, troubleshooting, and problem solving.