Spores of Clostridium novyi-NT, an attenuated strain of the obligate anaerobe Clostridium novyi, with potential immunostimulating and oncolytic activities. Upon intravenous administration, Clostridium novyi-NT spores germinate exclusively in hypoxic tissue, such as avascular regions of tumors. Germination results in lysis and destruction of surrounding viable tumor cells. Although C. novyi-NT spores do not proliferate in oxygenated tumor regions, they may stimulate the immune system to mount a cytolytic immune response against tumor cells, both hypoxic and well-oxygenated.