To investigate the bactericidal activity of gatifloxacin in serum and urine against relevant pathogens. Serum and urine samples obtained in a single rising dose pharmacokinetic study were investigated for bactericidal activity. The doses employed were placebo, and 200, 400, 600 and 800 mg of gatifloxacin. The titers obtained reflected the dose and susceptibility of the pathogen. In serum a titer of ≥ 1:8 was observed for Escherichia coli for >24 h after 400 mg; for Staphylococcus aureus and Acinetobacter baumanii the titer was ≥1:8 for between 12-24 h after 600 mg; for Serratia marcescens it was ≥1:8 for >12 h following 800 mg; and for Streptococcus pneumoniae it was ≥1:8 for 8 h following 800 mg. In urine, the titers were greater, and for Escherichia coli, Staphylococcus saprophyticus, Proteus mirabilis and Enterococcus faecalis the titer was > 1:8 for 12-24 h after 200 mg and for 24-36 h after a 400-mg dose. Pseudomonas aeruginosa demonstrated a titer of >1:8 for 18-24 h following 600 mg. These data suggest that gatifloxacin should be efficacious for a wide range of systemic and urinary tract infections.