There are at least two lines of dogs competing for the breed name American Mastiff; neither is recognized by any major English-language kennel club.

One line is the dry-mouthed American Mastiff, developed by the Flying W Farms kennel by crossing the English Mastiff with an Anatolian Shepherd. This resulted in a breed that looks exactly like the English mastiff but without the drooling that is common in the English Mastiff. This American Mastiff loves children and are devoted to their families. These are loyal and calm dogs. They are generally nonaggressive except when their family is threatened. In those instances they become the fierce and courageous guard dogs that they are.

The second line is sometimes called the Panja American Mastiff, developed by the Panja kennel.

Both lines are very new with not much breed history, making them ineligible for registration in most breed registries; registries with relaxed requirements may recognize a new breed with minimal history, such as the Continental Kennel Club's recognition of the Flying W Farms line. Some critics claim that these are simply crossbred dogs or variants on the standard English Mastiff. It is not clear which line, if either, will ultimately become a solid breed known as the American Mastiff.