In order to estimate the effectiveness of sodium dichloroisocyanurate (SDI), an effervescent disinfectant tablet, for general disinfection use at medical facilities, its antimicrobial activities against various organisms, including hepatitis B virus, and its chemical stability were studied by comparing it with those properties of sodium hypochlorite (SHC). Solutions of SDI and SHC containing 100 ppm, 500 ppm and 1000 ppm available chlorine (av. Cl) showed equivalent activity against vegetative bacteria, Mycobacteria, fungi and bacterial spores. Virucidal action required a higher concentration of both disinfectants : 1000 ppm av. Cl of SDI and SHC did not inactivate hepatitis B surface antigen (HBs-Ag) in 60 min, whereas 5000 ppm av. Cl of both disinfectants inactivated HBs-Ag in 3 min at 25°C. Although solutions of SDI and SHC were relatively stable for 24 h under clean conditions, both solutions markedly decomposed in the presence of human serum. In the presence of 30% human serum, 10000 ppm av. Cl of SDI and SHC decomposed to approximately 6600 ppm and 2600 ppm, respectively, immediately after preparation at 25°C. Based on these results and the simplicity of preparation of dilutions for use, SDI is considered to be a useful disinfectant for use in medical facilities.