Two separate studies, one with pathogen-inoculated product and one with noninoculated product, were conducted to determine the safety and spoilage characteristics of modified atmosphere packaging (MAP) and traditional packaging of ground beef patties. Ground beef patties were allotted to five packaging treatments (i) control (foam tray with film overwrap; traditional), (ii) high-oxygen MAP (80% 02, 20% CO2), (iii) high-oxygen MAP with added rosemary extract, (iv) low-oxygen carbon monoxide MAP (0.4% CO, 30% CO2, 69.6% N2), and (v) low-oxygen carbon monoxide MAP with added rosemary extract. Beef patties were evaluated for changes over time (0, 1, 3, 5, 7, 14, and 21 days) during lighted display. Results indicated low-oxygen carbon monoxide gas flush had a stabilizing effect on meat color after the formation of carboxymyoglobin and was effective for preventing the development of surface discoloration. Consumers indicated that beef patties packaged in atmospheres containing carbon monoxide were more likely to smell fresh at 7, 14, and 21 days of display, but the majority would probably not consume these products after 14 days of display because of their odor. MAP suppressed the growth of psychrophilic aerobic bacteria when compared with control packages. Generally, control packages had significantly higher total aerobic bacteria and Lactobacillus counts than did modified atmosphere packages. In the inoculated ground beef (approximately 10(5) CFU/g) in MAP, Escherichia coli O157 populations ranged from 4.51 to 4.73 log CFU/g with no differences among the various packages, but the total E. coli O157:H7 in the ground beef in the control packages was significantly higher at 5.61 log CFU/g after 21 days of storage. On days 14 and 21, the total Salmonella in the ground beef in control packages was at 5.29 and 5.27 log CFU/g, respectively, which was significantly higher than counts in the modified atmosphere packages (3.99 to 4.31 log CFU/g on day 14 and 3.76 to 4.02 log CFU/g on day 21). Data from these studies indicate that MAP suppresses pathogen growth compared with controls and that spoilage characteristics developed in MAP packages.