Infectious coryza, a respiratory disease of chickens, is caused by the bacterium Haemophilus gallinarum. The disease has continuously plagued sections of the poultry industry in California for a number of years. Since prevention and control measures have met with relatively little success, new approaches have been made to the study of the disease. Haemophilus organisms associated with recent field cases of coryza and airsac disease have been isolated and characterized for their cultural aspects, biochemical activities, specific antibodies formed against them, and their virulence in causing disease in chickens, chicken embryos, and mice. Similar studies have been made on other microorganisms found associated with H. gallinarum in diseased tissues, especially members of the genus Pasteu-rella. Whether the severe disease observed in field cases was caused by H. gallinarum alone or by a combination of agents is under investigation.