An antibiotic substance was produced in a minimal medium added with seleno-DL-methionine by a marine Bacillus sp. No. 14¹⁾. We characterized its activity, structure and production conditions. The antibiotic showed a specific antibiotic activity to Micrococcus luteus and was found to be selenohomocystine of which antibiotic activity has never been reported. The antibiotic activity was also formed from seleno-DL-ethionine but not from seleno-cystine or inorganic selenium compounds.