An affinity chromatography method, using double stranded DNA, is described for the isolation of a deoxyribonuclease from the medicament Varidase, a wound cleansing preparation produced from the extracellular products of a species of Streptococcus. Using a complex buffer system, which includes EDTA, mercaptoethanol, bovine serum albumin and 50 mM NaCl, conditions are produced which allow binding of the nuclease to the chromatographic matrix without apparent degradation of the DNA. Increasing the NaCl concentration to 2 M results in the elution and isolation of a relatively non-specific endonuclease with a molecular mass of approximately 38 000 and a pI of 4.6. Experiments using ammonium sulphate for partial fractionation of the Varidase proteins, prior to chromatography, improved the yield but were not essential for obtaining a purified product. It is suggested that this technique should be readily applicable for the isolation of deoxyribonucleases from other sources.