This initial study with ETI-204 will examine the safety and pharmacokinetics of ETI-204 following IV administration both in the absence and presence of ciprofloxacin. The pharmacokinetics of ciprofloxacin in the presence of ETI-204 will also be determined. This study is a placebo-controlled, double-blind, randomized study which will be conducted at a single site. Healthy male and female volunteers will be recruited from the Columbus, OH area, and will have a maximum participation period of 7 months for each subject (from enrollment to end of follow-up period). The study population will roughly parallel the ethnicity of this geographic area. Because this is the first time that ETI-204 is studied in man, individuals under the age of 18 will not be permitted to participate in the study. Part 1 is a randomized, placebo-controlled, double-blind study being conducted at a single study site. Part 2 will be double-blind with respect to ETI-204 but not blinded for oral ciprofloxacin. Part 1 of the study is the dose escalation portion, in which the safety, tolerability, and pharmacokinetics of single IV doses of ETI-204 will be evaluated. Three dose levels are proposed for study: 18 mg, 54 mg, and 108 mg. At each dose level, 3 males and 3 females will be randomly assigned to active drug and 1 male and 1 female will be randomly assigned to placebo. Thus, at least 24 subjects will be studied in Part 1. Part 2 of the study will determine the safety, tolerability and pharmacokinetics of a single IV dose (108 mg dose from part 1, presuming no safety or tolerability issues), and whether or not there is an effect of ETI-204 on ciprofloxacin pharmacokinetics. There will be 2 groups of 6 subjects each. The first cohort will receive ETI-204 plus ciprofloxacin and the second will receive ETI-204 placebo plus ciprofloxacin. Together with the comparable group from part 1, this will allow the determination of the effects of ciprofloxacin on ETI-204 pharmacokinetics and of ETI-204 on ciprofloxacin pharmacokinetics. Subjects will receive ciprofloxacin 500 mg or placebo every 12 hours for a total of 14 days beginning at the time of ETI-204/placebo administration. It is anticipated that the number of subjects in part 1 of the study (18 receiving active ETI-204 and 6 receiving placebo) will be sufficient to allow a preliminary assessment of safety and efficacy (ETI-204 concentration). In part 2, 12 subjects will receive active ETI-204 (108 mg dose from part 1, presuming no safety or tolerability issues). Of these, 6 will also receive oral ciprofloxacin and 6 will serve as the controls (i.e., ETI-204 alone). An additional cohort of 6 subjects will receive ETI-204 placebo and oral ciprofloxacin. This number of subjects should be sufficient to provide an indication of the presence of a drug-drug interaction between ETI-204 and ciprofloxacin, but not necessarily the magnitude. All statistical comparisons will be performed using two sided tests at an alpha level of 0.05 unless specifically stated otherwise. All statistical null hypotheses assert that there are no differences between treatments. Because this is the first study in man with ETI-204, the full risks can not be determined. ETI-204 is being tested for its utility in preventing the life-threatening sequelae that may occur following exposure to anthrax. The results of this study could demonstrate that ETI-204 has an acceptable safety profile and that it has a pharmacokinetic profile that will provide for serum concentrations of the drug (antibody) over time that will be effective in preventing the toxicity of one of the exotoxins produced by B. anthracis.