In the 19th century, Japan was a country completely isolated from the entire world, so there was no question of cross-breeding. Only in 1854, the borders were opened and European fighting dogs such as Bulldogs (1872), Mastiffs (1874), German Pointers (1876), Great Danes (1924) began to arrive in the Japanese Isles.
The breeding of Tosa was based on local so called Dogge dogs, probable descendants of ancient Molossian type dogs, as well as fighting dogs of Shikoku-Ken (the breed existed only in Japan) and such imported breeds as Bull Terriers, Mastiffs, Bulldogs, Great Danes, Pointers, St. Bernards and Bloodhounds. There's now accurate data about the breeds which took part in creating of Tosa as the information is keptcompletely in secret . Hypotheses are built on the basis of the external similarity of the breeds.