Natural characteristics (chestnuts, trichoglyphs, white markings, and blood typing) and applied marks (tattoos, freeze marks, hot iron brands, and laser marks) used by horse breed registries were evaluated to determine the alterability. The results show that traditional methods: chestnuts, white marks of signalment, coat color brands and tattoos are not reliable. For example, the chestnuts (horny growths of vertical fibrous tissues on the inside of horse's legs) are amenable to surgical intervention for fradulent alteration of identity, and white marks of signalment (natural markings) are easily dyed. Brands denote ownership only, and that often changes. The best method of identification is the use of freeze marks, which painlesly establish a mark that has permanent production of white hair, a permanently bald area where the skin becomes darker, or a bald, dark-skinned area with white hair edges. This method, augmented by the use of tricholgyphs and blood typing, is used by the International Identification System (IIS). This system uses angle numerical and angle alphabetical symbols which are patented and internationally copyrighted, with their use licensed only for official programs. IIS allows enough numbers to have a unique mark for each horse in the world and is adaptable to computer data retrieval. Stolen horses that bear the visible, individualizing symbols of the IIS are eligible for inclusion in the National Crime Information Center (NCIC) computer system, established by the Federal Bureau of Investigation. The IIS should be used nationwide to monitor slaughterhouses, transportation of horses across State borders and international boundaries, operation of race tracks and horse shows, certification of insurance claims, and participation in equine health programs. In addition, selected law enforcement personnel should be trained as identifiers in the freeze-mark method. Future research should concentrate on studying the effectiveness of the positive horse identification system in the reduction of the slaughter of stolen horses, the reduction of fraud at horse shows and race tracks, and the recovery of stolen horses. Countries which adopted the IIS system should be studied. Description of the IIS (with figures), photographs, and 12 references are included.