The War of the Gargantuas(1966)

Part of the Frankenstein (Toho) Collection

During WWII, a human heart taken from Dr. Frankenstein's lab and is kept in Japan where it gets exposed to the radiation of the bombing of Hiroshima. The heart grows in size, mutates and eventually becomes an indestuctable humanoid. Later, a reptilian monster goes on a rampage. Eventually the Frankenstein creature and the reptile face off in a terrible battle. An experimental lab animal called a gargantua escapes from his captors and is suspected to be the creature that is killing people all over the countryside. But when the gargantua from the lab appears at the same time as the evil gargantua, the two begin to battle across Japan.