In October 2001, public health departments across the United States were part of an intensive response to a bioterrorism event using anthrax spores delivered by mail. It is useful to examine this experience as an unscripted exercise of bioterrorism response capacity, more realistic than scenarios of planned exercises. The event particularly challenged public health laboratory and communications capacity, but it also tested surveillance and training capacity. The bioterrorism response demonstrated the importance of strong partnerships between the public health laboratory and emergency response agencies as well as medical providers and the usefulness of open, flexible communication strategies.