This research was conducted to investigate the theory that the natural flora of ground beef hinders or even is lethal to the growth and survival of E. coli O157:H7. The complex idea of bacterial competition or the production of natural antibacterial substances by the natural flora found in ground beef is possibly a deciding factor in the low percentage rate of E. coli O157:H7 found in retail ground beef supplies. To investigate these theories, the natural Enterobacteriaceae of raw ground beef were screened for inhibitory activity and the ability to produce antimicrobial substances. Previous research has found that two main bacteria show inhibitory effects on E. coli. These are Pseudomonas aeruginosa and Serratia liquefacians and they naturally occur in the alimentary canal of animals and are part of the natural flora in ground beef. Ground beef was inoculated with E. coli O157:H7 at a level of 1000 cells/gram and stored at refrigerated temperatures. The ground beef was sampled daily for the survival of the pathogen and after extended storage, it was found that the levels of E. coli remained constant. When E. coli was introduced into previously irradiated ground beef containing only the P. aeruginosa and S. liquefacians, different growth characteristics were found. Ground beef containing P. aeruginosa did not inhibit pathogen growth and in fact, E. coli numbers increased. But when S. liquefacians was introduced, the numbers of E. coli began to decline, suggesting that the S. liquefacians strain was outcompeting the E. coli O157:H7 population. Based on these results, purified forms of the antimicrobial substances were extracted and introduced into ground beef containing the pathogen. It was difficult to purify the antimicrobials in this study, and in fact, the more purification the less the inhibitory effect. Nonetheless, crude extracts of the antimicrobial portions were introduced into inoculated ground beef samples at 1, 5 and 10% levels. At refrigerated storage, the control inoculated ground beef had increases in E. coli of about 1 log, similar to the sample that was inoculated at the 1% level. At the 5 and 10% level, the antimicrobial did exhibit a bacteriostatic effect on the growth of E. coli with the inoculated culture not able to increase in numbers. This study indicates that there needs to be a higher level of antimicrobial purification and that there is probably more than one inhibitory substance being produced, possibly proteolytic and lipolytic enzymes.