A rapid and simple procedure is described for obtaining CsCl-purified DNA from multiple small samples of cells or tissue. The DNA is recovered in a high-molecular-weight form (greater than or equal to 50 kb) that is readily cleaved with restriction enzymes. Sufficient quantities of DNA (10-50 micrograms) are recovered to allow multiple analyses by Southern blotting and most cloning procedures. The isolation procedure involves addition of intact cells or powders of frozen tissues directly to a simple lysis buffer containing detergent (sodium dodecyl sulfate or sodium sarcosinate) and high concentrations of EDTA. Ultra-high-speed centrifugation of CsCl gradients allows the isolation of DNA from 10 different samples in as little as 5 h. Applications are described for mammalian cells (HeLa cells), insect tissues (Drosophila melanogaster adults and pupa, Manduca sexta pupa, and Musca domestica pupa), higher plant tissues (Vicia faba leaves and meristems), algal cells (walled and wall-less Chlamydomonas reinhardi), yeast cells (Saccharomyces cerevisiae), and bacterial cells (Escherichia coli spheroplasts for preparation of both chromosomal and plasmid DNA). The procedure can be scaled up with larger sample sizes and longer centrifugation times to provide bulk quantities of DNA.