In the United States, Grade A milk, also called fluid grade milk, refers to milk produced under sufficiently sanitary conditions to qualify for fluid (beverage) consumption.
Grade B milk (also referred to as manufacturing grade milk) does not meet fluid grade standards and can only be used in cheese, butter and nonfat dry milk.
More than 40% of all milk produced nationally is Grade A, and much of the Grade A milk supply is used in manufactured dairy products.
This article incorporates public domain material from Jasper Womach.
Report for Congress: Agriculture: A Glossary of Terms, Programs, and Laws, 2005 Edition (PDF).