Plasmid purification by alkaline lysis is one of the most generally used molecular biology techniques. Traditionally, the cleanest plasmid DNA has been made by banding the DNA on a cesium chloride gradient. This, however, is time-consuming and expensive, so alternative plasmid purification methods have been sought. One that is becoming more widespread is the use of silica-based DNA-binding matrices. Such matrices are available commercially, but can also be easily made in the laboratory. In this chapter, a protocol for large-scale plasmid DNA preparation using diatomaceous earth is presented. This protocol is based on methods of Boom et al. (1 ) and of Carter and Milton (2 ), as well as recommendations for homemade DNA purification kits in TIBS (3 ). Although this protocol is a large-scale preparation, “maxiprep” and yields milligrams of DNA, it can also be adapted for the small-scale, “miniprep” (2 ) to yield microgram quantities,