On June 27, 2011, a worker at a poultry processing plant in Arkansas began to pour sodium hypochlorite into a 55-gallon drum that contained residual acidic antimicrobial solution. When the sodium hypochlorite reacted with the solution, greenish-yellow chlorine gas was released into the small room where the drum was located and then spread into the plant, where approximately 600 workers were present. These workers promptly were evacuated. Chlorine is a respiratory irritant and can produce symptoms ranging from mild eye, nose, and throat irritation to severe inflammation of the lung, which can lead to death. Of the approximately 600 workers who were evacuated; 545 were later interviewed, 195 reported seeking medical treatment, 152 reported being hospitalized, and the plant nurse reported that five were admitted to intensive-care units. The next day, the Occupational Safety and Health Administration (OSHA) asked for technical assistance from CDC's National Institute for Occupational Safety and Health (NIOSH) to evaluate health effects of the release and make recommendations to prevent future occurrences. This report describes the results of that evaluation, including findings from two follow-up site visits conducted approximately 4 and 6 months after the release. Of the 545 workers who participated in the evaluation, three developed reactive airways dysfunction syndrome (RADS), an irritant-induced form of asthma that can persist for life. The worker who inadvertently mixed the two solutions indicated that the drum was labeled in English but he could only read Spanish. This incident underscores the danger posed by chlorine gas and the importance of employers providing adequate training and communication of health and safety precautions to employees.