U.S. Customs agents yesterday seized 10 pounds of cocaine said to have a street value of $2.5 million, on a Braniff International jet at Dulles International Airport.

The haul is one of the largest amounts of cocaine recovered here in recent years, according to Customs spokesman Jim Mahan. He said the drug was found on a plane from South America -- a once-a-week flight that investigators call the "cocaine express" because they believe it is regularly used by cocaine smugglers.

Investigators discovered cocaine on the flight three weeks in a row last year at Dulles, Mahan said. The flight begins in La Paz, Boliva, stopping in Lima, Peru, and Dulles, before ending in New York.

South American is a major source of the cocaine used in the United States.

No arrests were made in connection with yesterday's seizure.

When Customs agents made a routine inspection of the plane shortly after it arrived at 9:10 a.m., a small bag of cocaine fell out of an overhead baggage compartment as it was opened, the spokesman said. A specially trained narcotics dog was brought aboard, the spokesman said, and it promptly sniffed out the bulk of the cocaine, which was hidden behind a false ceiling.

When the ceiling panel was removed, agents found the drug wrapped in a plastic bag.

Mahan said drug couriers, hired to travel between the United States and South America, usually hide the drugs on the plane and the package is retrieved by someone else after the aircraft empties at Kennedy International Airport in New York.

He said the cocaine found yesterday was "90 percent pure" and would have retailed on the street for nearly $256,000 a pound.

The seizure was the second major drug confiscation in the Washington area this week. Federal drug agents made what might have been one of the largest seizures of uncut heroin here when they confiscated nearly seven pounds of the drug Wednesday. The heroin, which was said to have a street value of $32 million, had been smuggled directly from an Iranian heroin processing plant, narcotics agents said. Three men, including the owner of a downtown Washingotn restaurant, were arrested in that case.