ANY SERIOUS discussion of currency reform must address the elimination of the penny and the half dollar.

Removing pennies from cash registers would make room for a $1 coin and produce numerous other benefits, including increased productivity. The National Association of Convenience Stores estimates that an average of two seconds is spent handling pennies during each of its members' 10 billion annual cash transactions -- a total of 5.5 million hours costing an estimated $22 million.

The United States has been minting about 50 pennies per person annually, a staggering number considering that they are no longer used in coin-operated machines. Three-fourths of the U.S. Mint's output is pennies. A March 1986 study conducted at Disney's Epcot Center reported that adults had an average of 993 pennies at home; children under 18 had 1077 pennies.

Billions more are thrown away each year. One Florida solid-waste recovery plant alone -- which processed 8,000 tons of waste weekly from about 650,000 residents in Dade County -- collected between $50,000 and $60,000 in loose change every year between 1983 and 1985. About 80 percent of that was pennies. The Epcot study reported that 6 percent of adults between 18 and 34 freely admit to throwing them away.

What would it take to eliminate this obsolescent coin? One solution would be to legalize rounding up or down to the nearest nickel on cash transactions. (Payments by check or credit card would not be affected.) The rounding would occur after all purchases were totaled and sales tax added. Because merchants pay sales tax on monthly aggregate sales, taxing authorities would receive exactly the same revenue. Customer and merchant would stand an equal chance of gaining or losing a maximum of two cents.

Critics might complain that eliminating the penny would force prices up. But there is no "mill" (thousandth of a dollar) coin; yet we continue to price gasoline, bulk mail and stocks to the mill and round them without inflationary effect. Besides, the penny would not have to be banished or demonetized. Its use would simply atrophy, such as occurred when the U.S. stopped making half-cent coins in 1857. Moreover, to maintain the integrity of the currency system, all coins and paper money ever issued by the government remain legal tender.

As for the 50-cent piece, the public has already rejected it. No wonder: The coin is exactly twice the weight of the quarter, so there is no "pocket" economy in carrying it; and coin-operated machines won't accept it. The Mint will not issue any Kennedy half-dollars this year, owing to lack of demand.