The efficacy of a florfenicol premix was studied in weaning pigs experimentally inoculated with Actinobacillus pleuropneumoniae. Twenty five clinically healthy pigs were distributed into 3 groups; group A non-medicated, groups B and C orally medicated with 20 and 40 ppm of florfenicol respectively. The pigs were fed during 12 consecutive days and on day 5 all the groups were challenged with A. pleuropneumoniae serotype 1. All the animals in Group A developed clinical signs. Most of the pigs in the medicated groups maintained a good health status. Postmortem examination revealed severe pleuropneumonia in pigs from the control group and pneumonic lesions in 40% of the animals treated with 20 ppm of florfenicol. Development of pleuropneumonia was prevented in all the pigs medicated with 40 ppm of florfenicol. Actinobacillus pleuropneumoniae was recovered from the lungs of all control animals and from one pig of each of the medicated groups, however, the avidin biotin peroxidase (ABC-P) method detected the presence of the microorganism in all the animals. We demonstrated that medication with feed containing 40 ppm of florfenicol blocked efficiently the signs and lesions caused by A. pleuropneumoniae and increased the daily body weight gain.