Technical Abstract: The ability of C. perfringens to germinate and grow in acidified ground beef as well as in ten commercially prepared acidified beef, pork and poultry products was assessed. The pH of ground beef was adjusted using organic vinegar to achieve various pH values between 5.0 and 5.6; the pH of the commercial products ranged from 4.74 to 6.35. Products were inoculated with a cocktail of three strains of C. perfringens spores to achieve ca. 2 log (low) or 4 log (high) inoculum levels, vacuum-packaged, and cooled exponentially from 54.4 to 7.2C in 6, 9, 12, 15 18 or 21 h, to simulate abusive cooling from the USDA-FSIS recommended cooling time of 6.5h. Total germinated C. perfringens population was determined after plating on tryptose-sulfite-cycloserine agar and incubating the plates anaerobically at 37C for 48 h. Also, C. perfringens growth from spores was assessed at an isothermal temperature of 44C. Growth from spores was inhibited in ground beef with a pH of 5.5 or below, even during extended cooling from 54.4 to 7.2C in 21 h. In ground beef with a pH of 5.6, the growth was more than 1- log after 18 h cooling from 54.4 to 7.2C. However, 15 h cooling controlled the growth to less than 1 log, regardless of the inoculum level. Also, no growth was observed in products with pH ranging from 4.74 to 5.17, both during exponential abusive cooling periods of up to 21 h and during storage for 21 h at 44C. While less than 1 log growth of C. perfringens from spores was observed in the pH 5.63 product cooled exponentially from 54.4 to 7.2C in 15 h or less, product with pH 6.35 supported growth even during 6 h cooling from 54.4 to 7.2C. These challenge tests demonstrate that pH adjustment of ground beef to pH less than or equal to 5.5 and of barbeque products to pH less than or equal 5.63 inhibits C. perfringens spore germination and outgrowth during extended cooling periods from 54.4 to 7.2C up to 15 h. Therefore, safe cooling periods for products with homogeneous, lower pH can be substantially longer.