The purpose of this study is to evaluate the safety and pharmacokinetics (how long a drug stays in the bloodstream and how high the levels of the drug are at different times) of ETI-204 following intravenous (IV-into a vein) injections, and to evaluate the effects that ETI-204 may have on the pharmacokinetics of oral (by mouth) ciprofloxacin, an antibiotic approved by the U.S. Food and Drug Administration (FDA). ETI-204 is an experimental drug (not approved by the FDA) intended to protect against anthrax (a bacterial infection). Approximately 36 male and female healthy volunteers ages 18 to 50 will be in this study. Participation in this study may last up to eight weeks. Volunteers will have a single IV dose of the ETI-204 study drug or placebo (inactive substance, and some participants will also receive ciprofloxacin. They will stay in the Clinical Pharmacology Unit at least 36 hours after the dose.