AbstractAditoprim (ADP) has potential use as an antimicrobial agent in animals. However, its pharmacodynamic properties have not been systematically studied yet. In this study, thein vitroantibacterial activities of ADP and its main metabolites were assayed, and thein vivoantibacterial efficacy of ADP for the treatment of swine streptococcosis was evaluated. It was shown thatSalmonellaandStreptococcusfrom swine,Escherichia coliandSalmonellafrom chickens,E. coli, Streptococcus, Mannheimia, Pasteurellafrom calves,StreptococcusandMannheimiafrom sheep, andE. coli, Flavobacterium columnare, Acinetobacter baumanniiandYersinia ruckerifrom fishes were highly susceptible to ADP.Haemophilus parasuisfrom swine,Staphylococcus aureus, Aeromonas punctate, Mycobacterium tuberculosis, Streptococcus agalactiaefrom fishes, andKlebsiellafrom calves and sheep showed moderate susceptibility to ADP, whereasE. coli, Actinobacillus pleuropneumonia, Pasteurella, S. aureus, Clostridium perfringensfrom swine,S. aureus, C. perfringensfrom chickens, andS. aureusfrom calves were resistant to ADP. The main metabolites of ADP showed equal activity to that of their parent compound, and the prevention and therapeutic dosages of ADP recommended for swine streptococcosis were 10 and 20~40 mg/kg b.w., respectively. This study firstly showed that ADP had strong antibacterial activity and had potential to be used as a single drug in the treatment of bacterial infectious diseases.