Upon fertilization, sea urchin eggs (Stronglyocentrotus pupuratus) release a protease into the surrounding sea water. This protease is in a particulate form which can be solubilized. The soluble form was purified by affinity chromatography on columns of immobilized soybean trypsin inhibitor. The purified enzyme is similar to bovine trypsin both in molecular weight (22500) and in susceptibility to inhibitors such as diisopropyl phosphofluoridate and soybean trypsin inhibitor. In contrast, extracts of unfertilized eggs appear to contain an inactive form of the enzyme which can be activated by dialysis at pH 4.6. The enzyme, as purified from extracts activated in this manner, was similar in its properties to that from fertilized eggs.