Seven species of bacterial biothreat agents were tested for susceptibility to UV light (254 nm). All gram-negative organisms tested required <12 mJ/cm(2) for a 4-log(10) reduction in viability (inactivation). Tailing off of the B. anthracis spore inactivation curves began close to the 2-log(10) inactivation point, with a fluence of approximately 40 mJ/cm(2), and 3-log(10) inactivation still was not achieved with a fluence of 120 mJ/cm(2).