D.C. police seized 5 1/2 pounds of heroin on Monday-the largest quantity of the drug ever confiscated in the city-after a security guard made a routine search of a shopping bag that a woman was carrying out of the U.S. Postal Service headquarters at L'Enfant Plaza.

Police said the heroin seized from the woman, and an additional amount seized later from a man identified as her common-law husband, was worth at least $109,000. The heroin was packaged in small quantities in waxed paper bags and apparently was ready for street sales, the police said.

They charged Mary K. Frye, 36, a secretary at the Commerce Department, and Robert L. Cunningham, 37, a printer at the Public Broadcasting Service, with possession of herion with intent to distribute it. The couple lives at 15 Cindy La., Seat Pleasant.

D.C. narcotics detective David E. Cassidy a shopping bag, was stopped by security Frank Scarborough as she tried to walk out of the postal service at 475 L'Enfant Plaza aout 4.40 p.m. Monday.

Cassidy said Scarborough saw "many waxed paper bags that appeared to contain white powder" and called a postal service investigator. Cassidy said that he and another D.C. narcotics detective, Michael Marcum, tested the powder and found that it was "a narcotic drug of the opiate group."

Cassidy said that a man, later identified as Cunningham, was identified as the person Frye had been seen with in the postal service headquarters, where Cunningham works at PBS. Asked by Postal investigator Clyde Ross if he knew Frye, Cunningham was quoted by polic as saying. "She's my wife."

When asked if he had seen her Monday, Cunningham allegedly responded, "Yes, she came to get a package, according to Cassidy's affidavit.

Cassidy siad that Cunningham was holding a "dark green plastic bag" with 86 packets of the white powder when he was arrested. Cassidy said the heroin seized from Frye had a street value of $51,200, and that the drugs confiscated from Cunningham were worth $58,080. In addition, Cassidy said that $793 in cash was seized from Cunningham.

Sources familiar with drug seizures in the District said the amount confiscated was the largest quantity of heroin ever seized in the District. They also said that when the purity of the heroin eventually is determined, the street value may actually total $500,000.

Frye and Cunningham both make more than $11,000 in their respective jobs, according to an attorney, Kenneth Michael Robinson, who represented them at a bond hearing before U.S. Magistrate Jean F. Dwyer.

Assistant U.S. Attorney Bobara Liles asked Dwyer to set bond at $15,000 each for Frye and Cunningham. But the magistrate said that both were entitled under D.C. bail provisions to be released on personal recognizance, and they were preliminary hearing for Dec. 21.