The objective was to evaluate the efficacy of a single dose of danofloxacin (6 mg/kg bodyweight) given by the intravenous route for the treatment of acute bovine mastitis induced by intra-cisternal infusion of an Escherichia coli strain (26 cfu into one rear quarter of each cow). Twenty-three Prim'Holstein lactating cows were inoculated. To be challenged, the mammary glands had to be productive, free of pathogenic bacteria, and with somatic cell counts (SCC) of <200,000 cells/ml. The cows were treated on an individual basis when predetermined criteria involving both systemic and local clinical signs were satisfied. Allocation to treatment, danofloxacin or negative saline control, was performed according to a randomized treatment allocation plan. Monitoring during a 21-d period after inoculation included individual clinical examination, bacteriological examination and determination of SCC. Esch. coli was isolated from the milk of all inoculated quarters at the first milking post-inoculation and, together with reference to the clinical scores; the challenge was considered to be successful in 20 of the 23 cows. On study day 7 bacteriological cure rates with danofloxacin and saline control were 89% (8/9) and 44% (4/9) respectively. On days 14 and 21 all milk samples that could be collected were negative for Esch. coli in both groups of animals. Beneficial statistically significant differences were found at the end of the observation period (days 19-21 post treatment) between cows treated with danofloxacin and saline for SCC (P=0.0091) and earlier in the study for milk production (P=0.0003) and udder inflammation (P=0.004). Obvious beneficial trends were recorded in the danofloxacin group for rectal temperature, milk quality, general behaviour and appetite. Danofloxacin-treated cows showed statistically significant lower local clinical scores and a more rapid return to pre-inoculation values. It was concluded that systemically administered danofloxacin is effective in terms of bacteriological results, milk production and both systemic and local signs when used in the treatment of induced acute Esch. coli mastitis. Danofloxacin hastens recovery and return to productivity compared with potential self cure.