Two enzyme forms were isolated from the commercial preparation of extracellular endonuclease of Serratia marcescens strain B10 M1. The chromatographic and electrophoretic properties, isoelectric points and N-terminal amino acid residues are different for both enzymes. At the final step of the purification procedure including ion-exchange chromatography on phospho- and DEAE-cellulose columns the yields of nucleases Sm1 and Sm2 were 13% and 25%, respectively. No significant differences were found in the specific activities of nucleases Sm1 and Sm2 (3.6 x 10(6) and 4.0 x 10(6) un. act./mg of protein). A comparative analysis of tryptic nuclease hydrolysate peptides was carried out. The amino acid sequences of some polypeptide segments of the proteins were determined. The structural similarity of the enzyme was established and the amino terminal regions of the proteins were identified. The localization of the disulfide bonds in the molecules of the both nucleases was determined. The similarity of nucleases Sm1 and Sm2 strain B10 M1 to S. marcescens endonucleases obtained from other strains was demonstrated.