Paul Corbin gleefully slapped his West Palm Beach-to-Washington ticket in his hand yesterday as he waited for his luggage at Washington National Airport and declared: "Because of this new coupon thing I'm going to take my wife to Kenya late this year."

Corbin was one of thousands of East Coast passengers who discovered yesterday that their inexpensive, short-hop tickets on National Airlines qualify them to take a family member with them to certain destinations for free later this year on National or its corporate cousin, Pan American World Airways.

Under the program, which started Monday, any National ticket between Washington or New York and Florida sold between now and April 9 entitles the purchaser to a coupon that can later be used to purchase two tickets for the price of one. The 2-for-1 deal runs between May 15 and Dec. 15.

A booming underground market in the street sale of airline discount coupons sprang up last summer when both American Airlines and United Airlines issued such coupons to passengers.

Corbin, and Alexandria-based market research consultant, paid $99 for a ticket from Florida to Washington. He got his coupon on the return leg of the flight and plans to use it to take his wife to Nairobi. He will be able to buy two round-trip tickets to Kenya for $1,548, the price of one, instead of $3,096, the price of two.

"We are doing it to attract passengers," said Merle Richman, Pan Am's Washington spokesman. "We are currently running our international flights at about 60 percent of capacity, and we want to increase that. So far today, the phones have been swamped," he said.

The program has been made possible because National and Pan Am recently merged, and corporate executives see the program as one way of publicizing the merger as well as bringing in new business, Richman said.

The discount program got off to what some passengers at National Airport said yesterday was a confusing start because officials failed to inform them of the program, or state they should save their tickets to qualify for it. t

"I left my boarding pass on the plane," said District of Columbia resident Elsie Dubb. "They didn't say a thing about it."

Northern Virginia attorney John D. Grad looked around in confusion when told of the program. "My 3-year-old was playing with the boarding passes. I hope she still has them," he said.

One man left the airline terminal, chagrined when told about the program, because he had left both his tickets and boarding passes on his plane.

Passengers said signs announcing the programs were posted in Florida cities but there were no signs posted yesterday at National Airport. A flyer handed out to departing passengers told them to "save your boarding pass" and turn it in with the "passenger coupon," without stating that the "coupon" was actually a duplicate of the airline ticket.

"We are going to have some hitches in these early days. We will handle everything on a case-by-case basis," Richman said.

Richman said he received an inquiry yesterday from a representative of a gay liberation group who claimed the airline's policy of restricting the free ticket to members of the paying passenger's family was discriminatory against gays.

"This was a business decision, a promotional thing. We had to draw the line somewhere," Richman added.

Travel agent Cal Simmons, owner of the Cal Simmons travel agencies in Alexandria and Georgetown, said yesterday that an unmarried man and woman who claimed to be married could probably use the coupons together without detection. "We don't normally ask for proof of marriage," he said.

A spokesman for World Airways in Oakland Calif., said yesterday his airline has started giving $50 discounts to cross-country passengers who pay full fare for one part of a trip. He said it was "too early to tell if the discount is attracting more passengers."

Spokesmen for American and United airlines said they have no current plans to bring back their discount programs.

"We don't know how much of the business we got would have come to us without the discount coupons," said American Airlines spokesman Vincent J. Modugno. "But it was a great promotion," he said.