Washington's two largest grocery store chains joined forces yesterday against a D.C. City Council proposal that would require large grocery stores to display an individual price on almost every item on their shelves.

Grocery stores have been moving recently to a practice of placing a price only on the shelf directly below the item, thereby saving the cost of stamping each item with its sale price. Larger stores in the city and the area have begun computerizing their checkout lines by implanting on each item a computer code that registers a price stored in a central computer.

The proposed bill, introduced by council member Wilhelmina J. Rolark (D-Ward 8), chairwoman of the Committee on Public Service and Consumer Affairs, would require a return to individual pricing on most grocery store items, except for fresh produce and prescription drugs. The committee held public hearings on the proposal yesterday.

Representatives of consumer groups and two council committee members who support the bill -- Rolark and Hilda Howland M. Mason (Statehood-At Large) -- said that individual pricing gives consumers a better chance to get the most for their shopping dollar.

"On first glance, this bill might appear to be proconsumer," said David W. Rustein, vice president and general counsel fo Giant Food Inc. "[But] the new system is faster and more efficient."

Ernest G. Moore, public affairs manager for Safeway, said: "For the first time in supermarket history, the price of each item does not have to be stamped on that item. . . Needless to say, the marking in prices on every item in a supermarket" is both timeconsuming and costly.

Both industry representatives said that since they began computerizing in 1976, their firms have been able to reduce prices. They added that the consumer gets a detailed price listing on a computerized receipt.

But supporters of mandatory item pricing said that the computer that registers prices at checkout often make mistakes, and a consumer has no way to determine whether he has been overcharged.

Council member Mason said that she is concerned that she might be overcharged by the computer when she shops. "I just hold my breath and hope that it's right," she said. "They have to make mistakes."

Rolark said the she initially introduced the bill three years ago when Washington stores began their new computer programs, but she delayed action until now to see how the new procedure would work. "I'm wed to the idea that I want to know what I'm buying," she said.

Maryland and Virginia both have rejected similar item-pricing laws.