This experiment was carried out to study the effect of metal ions on the proteolytic activity of intracellular protease extracted from Lactobacillus bulgaricus CH-18. Intracellular protease of L. bulgaricus CH-18 was isolated and purified by means of column chromatography with DEAE-cellulose and Sephadex G-100. The proteolytic activity of intracellular protease was tested on 1.25% casein solution as a substrate. The results are summarized as follows; 1. The proteolytic activity of intracellular protease was accelerated by the addition of CO², Fe² and Mn². 2. In the addition of Ca², Mg² and Zn², the proteolytic activity of intracellular protease was not affected particularly. 3. The proteolytic activity of intracellular protease was lowered by the addition of Cu².