The antimicrobial activity of the culture fluid of Lentinus edodes mycelium grown in submerged liquid culture was tested against some common bacterial species and Candida albicans. The mycelium-free culture fluid was bacteriostatic against Streptococcus pyogenes, Staphylococcus aureus and Bacillus megaterium. The substance responsible for the activity was heat-stable, could be extracted with chloroform and had a molecular weight under 10000. These characteristics suggested that the component might be lenthionine, an antibacterial and antifungal sulphur-containing compound. The culture fluid was less toxic to human tissue culture cells than to microbes. The antibacterial activity and toxicity could not be attributed to the same component.