One hundred and ninety-six boneless beef roasts (longissimus muscles) were vacuum packaged. Twenty-eight roasts remained vacuum packaged to serve as controls while the remaining packages were injected with one of six gas mixtures: (a) 100% O2, (b) 20% CO2 + 80% N2, (c) 50% CO2 + 50%O2, (d) 20% CO2 + 80% O2, (e) 25% CO2 + 25% O2 +50% N2 or(f) 51% CO2 + 30% O2 + 18% N2 + 1% CO. Five cuts from each packaging treatment were stored in a 1-3-C cooler for 7, 14, 21, 28 or 35 days and subsequently evaluated for relative percentages of various gases in the intact packages, off-odor, surface discoloration. overall appearance, metmyoglobin percentages and palatability. Packages initially injected with modified gas atmospheres containing O2 increased in relative percentages of CO2 with advancing storage times. Roasts stored in modified atmospheres initially containing high levels of O2 exhibited a greater incidence of off-odor, more surface discoloration, lower overall appearance ratings, shorter retail caselife and lower overall palatability ratings than vacuum-packaged roasts or roasts stored in an atmosphere initially containing 20% CO2 + 80% N2.