Using DEAE-cellulose chromatography, three peaks of the DNA-polymerase activity were found in a homogenate of the sea urchin Strongylocentrotus intermedius embryos at stage 32 of the blastomer. The isolation and purification of DNA-polymerase making up the bulk of he DNA-synthesizing activity of the sea urchin embryo cells included fractionation by ammonium sulfate, chromatography on DEAE-cellulose, hydroxyapatite and DNA-cellulose, resulting in a 2750-fold purification of the enzyme. The enzyme activity requires the presence of two-chain DNA activated by pancreatic DNAse, four dNTP and Mg2+. The enzyme is inhibited by a high ionic strength (150 mM KCl or NaCL) and the sulfhydryl reagent--N-ethylmaleimide; the pH optimum is 8.0. The molecular weight of the enzyme as determined by gel-filtration is about 150 000. It is assumed that the enzyme under study can be related to DNA-polymerases of the alpha-type.