WASHINGTON (AP) _ The Agriculture Department announced Tuesday a one-cent increase in the price it pays for cheese to support the price of milk, the first official action under the new Food Security Act of 1985.

President Reagan signed the new farm bill on Monday. Most provisions will require some time to implement, including 1986 crop provisions.

Assistant Secretary Orville Bentley said the new law prohibits USDA from considering the market value of whey in determining milk supports.

Since April 1, the department's purchase price for cheese has been based on an estimated return of one cent per pound for whey powder, a byproduct of cheese-making.

With the elimination of the whey factor, the USDA purchase price for cheddar cheese produced on or after Dec. 23 will be increased one cent per pound, to $1.25 per pound for 40-pound blocks and $1.2075 per pound for 500- pound barrels.

The purchase prices for butter and non-fat dry milk remain the same.

Under the program, the department is required to buy surplus butter, cheese and non-fat dry milk to support the price of milk paid to dairy farmers.