Since 1952, when Colonel Harland Sanders opened his first franchise, only a select few have been privy to the secret "herbs and spices" contained in the billion-dollar blend.  To protect the top secret recipe, the company claims, portions of the secret blend are premixed at two confidential spice companies and then distributed to KFC's offices, where they are combined.  In 1983, in his book Big Secrets author William Poundstone hired a laboratory to analyze a dry sampling of the spice mixture.  The surprising discovery was that instead of identifying "eleven herbs and spices," the analysis showed only four ingredients: flour, salt, pepper, and monsodium glutamate, a flavor enhancer.

The cooking procedure is believed to be the other half of the secret.  Colonel Sanders became famous for using a pressure cooker shortly after its invention in 1939.  He discovered that hungry travelers greatly appreciated the ten-minute pressure-cooking process (compared to the half hour is used to take for frying chicken), and the new process made the chicken juicy and moist inside.

KFC is the third-largest fast-food chain in the country and uses around 500 million chickens every year.

from the book Top Secret Recipes by Todd Wilbur