Deoxyribonuclease I (DNase I) from rat urine was purified about 3,000-fold to apparent homogeneity with a 14% yield by affinity chromatography utilizing polyguanylic acid-agarose and DNA-cellulose. The purified enzyme preparation was found to contain no other detectable nucleases. Isoelectric focusing electrophoresis revealed that all six isoelectric forms of the enzyme had been purified, and the resulting bands all contained DNase I activity. Quantitative amino acid analysis and N-terminal amino acid sequencing were performed on the purified DNase I. The N-terminal sequence up to the 15th residue of the enzyme was identical to that of rat parotid DNase I. The enzyme was found to be a glycoprotein, containing 1 fucose, 10 galactose, 17 mannose, 12 glucosamine, and at least 3 sialic acid residues per molecule. The isoelectric multiplicity of the enzyme was partly due to differences in the sialic acid content of the isoforms. Gel filtration on Superose 12 and electrophoresis on sodium dodecyl sulfate polyacrylamide gels indicated an approximate molecular mass for DNase I of 32 kDa. The enzyme had an optimum pH of 6.5 and required divalent cations such as Ca2+ for its activity. Its activity was inhibited by 1 mM EDTA and EGTA, but not G-actin. An antibody against the purified enzyme was found to be monospecific against rat urine and the pure antigen, and completely blocked the activity of the purified enzyme.