Loss of the genetic determinant for pesticin I in Pasteurella pestis results in concomitant loss of the plague coagulase and fibrinolytic factor. The median lethal dose for mice of an isolate lacking only these activities is increased by factors of about 10(1), 10(4), and 10(7) cells when administered by the intravenous, intraperitoneal, and subcutaneous routes, respectively. Virulence of the aforesaid strain can be enhanced in mice treated with 40 microg of ferrous iron. This response resembles that of Pasteurella pseudotuberculosis, a closely related species that normally lacks pesticin I.