This report describes a reliable and inexpensive gradient former constructed from disposable plastic syringes. The dual-chamber device produces both linear and exponential gradients which are very reproducible and essentially identical to the theoretical curve. Two identical syringes which serve as a mixing chamber and reservoir chamber are heat sealed at the bottom and glued to a flat plastic support plate. Small holes are drilled into the side walls of the syringes to accommodate an outlet tube and mixing tube. To form linear gradients, equal volumes of solution are placed in both chambers. The solution in the mixing chamber is stirred with a small magnetic stir bar. As starting solution is drawn from the mixing chamber, ending solution is gradually drawn into the mixing chamber to produce a linear gradient. To form exponential gradients a syringe plunger is placed into the mixing chamber to form a fixed volume of starting solution. Ending solution is placed into the reservoir chamber and may be replenished at any time during gradient formation. This allows small gradient formers to produce large volume exponential gradients. These gradient formers, which require only about (10 \mathrm{~min}) to construct, are durable enough to be used many times and inexpensive enough to discard if desired. (\Subset 1994) Academic Press, Inc.