A repeated dose toxicity study of prulifloxacin, a new antibacterial agent, was conducted in beagle dogs. Male and female dogs were given the test material orally for 13 weeks at doses of 0 (control), 20, 100 and 500 mg/kg. After discontinuation of the treatment, a 5-week recovery test was also conducted. Vomiting, salivation and decreased body weight gain or reduced body weight were seen in the 100 and 500 mg/kg groups. In the 500 mg/kg group, tremor, paresis of posterior limb associated with prone or sitting position and decreased food consumption were also observed. There were no treatment-related effects on survival and water consumption. Ophthalmoscopic, electrocardiographic and hematologic examinations, and urinalysis failed to show any abnormalities attributable to the treatment. Blood chemical examination showed increased GPT and decreased beta- and gamma-globulins in the 100 and 500 mg/kg groups, and increased GOT in the 500 mg/kg group. In pathological examination, cavitations and erosions were seen in the humeral and femoral articular cartilages in the 100 and 500 mg/kg groups. The above-mentioned changes were satisfactorily reversible except for erosions in the humeral and femoral articular cartilages in the 100 and 500 mg/kg groups. No toxicological findings were seen in the 20 mg/kg group. The results show that the NOAEL of prulifloxacin is 20 mg/kg for 13-week repeated dose toxicity in dogs.