Cyanogen bromide (CAS No. 506-68-3) was evaluated for acute inhalation toxicity. The test substance was administered to groups of 10 male and 10 female Sprague Dawley rats. Concentration, duration, and mortality data are as follows: 10 ppm for 6-hours (0/8 M, 0/8 F); 35 ppm for 6-hours (0/8 M, 0/8 F); 45 ppm for 6-hours (8/8 M, 2/8 F); 56 ppm for 6-hours (8/10 M, 10/10 F); and 112 ppm for 2.5-hours (10/10 M, 10/10 F). There were no adverse effects at 10 ppm and transient nasal irritation at 35 ppm. There were no treatment-related lesions at necropsy below 35 ppm. Clinical signs above 35 ppm included moderate to severe respiratory tract irritation, with rats gasping for breath during and after exposure, and severe weight loss with a slow recovery. Necropsy findings revealed severe respiratory tract irritation, congested lungs and liver, and distended stomachs due to mouth breathing. The LC50 was determined to be 39.3 ppm for males and 52.7 ppm for females.