The DNase A protein of 'F. succinogenes' S85 may play a role in preventing DNA uptake by this ruminal bacterium. Based on zymographic analysis, this enzyme appears to be the only detectable non-specific endonuclease expressed by 'F. succinogenes'. Specific nuclease activity remains constant throughout all growth phases and nuclease activity is exhibited mainly within an extracytoplasmic extract from cells. DNase A was purified to homogeneity by a combination of ammonium sulphate precipitation and chromatographic methods. Kinetic parameters for DNase A were measured from the cleavage of herring testes DNA and the pure enzyme exhibited a KM of 61 ± 12 m M and a 'kcat' of 330 ± 25 s-1. DNase A therefore has a catalytic efficiency approximately three-fold greater than bovine pancreatic DNase I. The enzyme has a requirement for divalent cation co-factors. Magnesium at a concentration of 20 mM stimulates maximal activity and manganese can substitute for magnesium, but is less stimulatory. Calcium ions or monovalent cations do not stimulate activity. DNase A exhibits a temperature optimum of 25°C and a pH optimum of 7.0. The enzyme is strongly inactivated by diethylpyrocarbonate, but not by iodoacetamide or tetranitromethane. Activity can be partially recovered from acylated enzyme after treatment with hydroxylamine-HCl. DNase A possesses one or more disulfide bonds and the protein is inactivated in the presence of reducing agents with or without subsequent alkylation by iodoacetamide. DNase A accumulates as an oxidized species ' in vivo'. An extracytoplasmic location for the active species may allow the enzyme to intercept and degrade DNA that has passed through the outer membrane. (Abstract shortened by UMI.)