The value of prophylactic antibiotics in the treatment of animal bites is uncertain. A prospective double blind stratified trial was performed comparing a broad spectrum antibiotic amoxycillin/clavulanate with placebo in full thickness animal bite wounds in a series of 185 consecutive patients. In wounds less than 9 h old, no significant benefit was found with the antibiotic. In older wounds, presenting 9-24 h after injury, the infection rate was reduced significantly (P = 0.023), although the numbers were small. We recommend the use of such an antibiotic on wounds 9-24 h old and query its use in more recent injuries, unless tendons or joints are likely to be involved.