Growth of the obligate anaerobes Clostridium botulinum and C. sporogenes is inhibited by 0.02 per cent sodium nitrite in fish digest broth at acid pH, inhibition not being accentuated by brief exposure of the cultures to air during the vigorous growth phase. Sodium nitrite inhibits growth of facultative anaerobes in nutrient broth under both strictly and semi-anaerobic conditions. Growth of facultative anaerobes is inhibited by sodium nitrite over a much wider pH range in nutrient broth than in fish digest broth or in a simple synthetic ammonium lactate medium. Sodium nitrite is capable of acting in either bactericidal or bacteriostatic capacity under different conditions, and inhibits growth of the human pathogens Eberthella typhosa and Staphylococcus aureus. The experiments described throw no light on the mechanism by which sodium nitrite, or its decomposition products, inhibit bacterial growth.