The interaction between Ca2+- and Mg2+-dependent DNAse from Strongylocentrotus intermedius embryos with native DNA was studied by an immunological electron microscopy method. Colloidal gold-labelling was ascertained to be an endonuclease that reacted along the whole length of the native DNA chain. After the phosphodiester bond hydrolysis the DNA-DNAse complex did not dissociate. The enzyme remained was bound with the terminal DNA fragment. Kinetics of enzymatic hydrolysis of DNA suggests the presence in the molecule of DNA of some specific nucleotide sequences which are recognized and split by the enzyme.