Sodium hypophosphite was evaluated for inhibition of growth and toxin production of Clostridium botulinum in a model meat system. Fresh comminuted pork was formulated to contain 0, 1000, or 3000 ppm sodium hypophosphite, with or without 50 ppm sodium nitrite, and 156 ppm sodium nitrite. The test formulations were inoculated with an equal mixture of 9 different strains of C. botulinum spores, vacuum packaged, pasteurized, and incubated for 60 days at 27°C. Packages were removed from incubation as swells developed and were analyzed for botulinal toxin by the mouse bioassay. In the first experiment, first toxic samples occurred at 5, 12, and 60 days of incubation for 0, 1000, and 3000 ppm sodium hypophosphite, respectively. Combinations of 1000 or 3000 ppm hypophosphite with nitrite delayed toxic swells by 17 and 14 days respectively, as compared to 6 days for 50 ppm nitrite alone. An experiment on the effects of sodium chloride on sodium hypophosphite inhibition showed that as the sodium chloride level was increased from O to 1.25 to 2.5%, hypophosphite inhibition was enhanced. For any sodium chloride level tested, increasing the hypophosphite concentration, caused a reduction in the rate of toxic swell formation. A combination of 2.5% salt, 50 ppm nitrite, and 3000 ppm sodium hypophosphite was the most effective combination tested. First toxic swell was delayed to day 24, and subsequent swells to day 33. The effects of various divalent cations on hypophosphite inhibition were studied. Cations produced no effect on sodium hypophosphite inhibition of C. botulinum. All experiments showed sodium hypophosphite to delay swell and toxin formation caused by Clostridium botulinum. / M.S.