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Background:The interest in using bacteria as anti- cancer therapeutic agents dates back to the end of the19th century. Some bacteria like Salmonella and Listeria replicate effectively inside malignant cell lines and suppress their growth. The bacterium Streptococcus pyogenes has become medically famous as a flesh-eating pathogen since mid-1980s. It is the causative agent of a life threatening clinical condition called necrotizing fasciitis. S. pyogenes usually produces a range of lytic enzymes that promote bacterial pathogenesis. With these characters, could this bacteria. be employed as a curing agent for certain cancers? The aim of this study was to determine the influence of S. pyogenes on malignant cellular death (apoptosis or necrosis)- in an ex-vivo "experimental- interventional" study.Methods: The cytotoxicity of fifteen internalized streptococcal strains( including 12 clinical isolates, 2 known M types [M1, M3] and standard strain), on four types of malignant cell lines- A549, BT-20, PC-3, L-929- were tested by Trypan blue exclusion, DNAfragmentation and WST-1 methods. The streptococcal protease, lipase, DNase and serum opacity factor (SOF) were tested concurrently. The standard strain of Streptococcus (Enterococcus) faecalis was employed as negative control. The results were analyzed by statistical Minitab software.   Results: The overall cytotoxicity rate of -internalized- S. pyogenes was 57% by trypan blue method and 50 % by DNA electrophoresis. False positive results occurred for the negative control in WST-1; therefore this test did not present reasonable results. The correlation between production of SOF, lipase, DNase and cytotoxicity of S. pyogenes was not significant (p > 0.05). However, 67% of the protease positive strains induced cellular death in at least one type of - malignant cell line (p