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SUMMARY 1. The presence of extracellular DNA, but not RNA, at μg 1−1 concentrations was demonstrated in seawater, river water and in a series of eutrophic lowland ponds.

2. Extracellular DNA in natural waters invariably comprised two major components: one of high molecular weight (>20 kb) derived from viruses, and another of low molecular weight (1–500 bp) and apparently free in solution. Similar species of DNA molecules were produced within 2 weeks in simple laboratory modules.

3. A purification procedure was developed which separated extracellular DNA from other components of dissolved organic matter, and which also separated viral from soluble DNA. Both types of DNA were purified sufficiently to constitute active templates for enzymes used in DNA manipulation.