In June 1999, academic infectious disease experts, national public health experts, Department of Health and Human Services representatives, civilian and military intelligence experts, and law enforcement officials met to review and comment on the threat potential of various agents to civilian populations. The following general areas were used as criteria: (1) public health impact based on illness and death; (2) potential for delivery to large populations based on stability of the agent, ability to mass produce and distribute, potential for person-to-person transmission; (3) public perception as related to public fear and potential civil disruption; and (4) special public health preparedness needs. Based on the overall criteria and weighting, agents were placed in one of three priority categories for initial public health preparedness efforts. Agents in Category A have the greatest potential for adverse public health impact with mass casualties, and most require broad-based public health preparedness efforts. These agents (e.g., Variola major (smallpox), Yersinia pestis (plague), and Francisella tularensis (tularemia)) also have a moderate to high potential for broad dissemination or a heightened general public awareness that could cause mass public fear and civil disruption. Category B agents (e.g., Coxiella burnetii (Q fever), Burkholderia mallei (glanders), and Rickettsia prowazekii (typhus fever)) generally cause less illness and death and therefore would be expected to have lower medical and public health impact. They also require fewer special public health preparedness efforts. Category C is for biological agents that are currently not believed to present a high bioterrorism risk to public health but could emerge as future threats. Tables, appendix, references