Fairfax County police said yesterday they have traced about 70 burglaries to a 37-year-old real-estate agent who allegedly used his "lock box" key to enter houses and steal personal property totaling $300,000.

The burglaries took place in Arlington, Alexandria, Falls Church, Fairfax City and Fairfax County over the last six months at houses equipped with the boxes that contain house keys and allow entry to real-estate agents when the owners are not at home.

Police said Michael Montrose of 2758 Hyson La. in Falls Church was arrested Friday after a 15-year-old Springfield youth alerted police that a man was burglarizing his home, then let the air out of one of the tires on the suspect's car.

Montrose was apprehend after a 1 1/2 mile chase by Fairfax County police that ended when Montrose's car turned over and jewelry and other items spilled out at the intersection of Keene Mill and Rolling roads in Springfield.

Montrose, identified as a part-time agent for Century 21-Rowan and Estes Inc. of Falls Church, is being held in Fairfax County jail in lieu of $122,000 bond, according to police. He has been charged with three burglaries, reckless driving and speeding to elude an officer, and more charges are expected, police said.

At a press conference yesterday, Cpl. Andrew Page, who headed the investigation into the burglaries, said police had recovered an estimated $100,000 to $3000,000 worth of property from Montrose's car, his residence and a locker Montrose allegedly rented in Alexandria.

Included in the recovered items, shown to reporters yesterday, were six fur coats, six Selectric II IBM typewriters, silverware, jewelry, suitcases, tape recorders and microscopes. Page said police still are looking for as much as 30 percent of all the items stolen.

Page said Montrose intended to sell the merchandise and in fact had sold some before his arrest, but he refused to elaborate.

Only agents registered with a realty firm may obtain the key that unlocks a lock box, according to Arthur W. Kogstad, executive vice president of the Northern Virginia Board of Realtors.

Kogstad, who said the lock box is a "very valuable marketing device," estimated that between 4,800 and 5,000 homes up for sale in Northern Virginia have lock boxes installed at any one time. Kogstad emphasized that use of the device was strictly up to the homeowner.

Some Northern Virginia owners, aware of the break-ins, have refused the boxes in recent weeks, according to real-estate agents in the area.

Kogstad added that locks on all the boxes in Northern Virginia had been changed nine months ago as part of a routine security process. Kogstad said new keys are issued to agents every 2 1/2 to three years.

Fairfax police said Montrose is from Los Angeles, where he worked in the computer business, and that he moved to the Washington area last September. Montrose has been associated with Century 21-Rowan and Estes as a part-time agent since the end of January, according to a company spokesman who asked not to be identified.

The Estes spokesman described Montrose as "a very intelligent individual" but also one who spent little time at his job. Since starting work at the firm, the spokesman said, Montrose had not sold any houses and came to the office once or twice a week.

The spokesman said Montrose's license was returned to the Virginia Real Estate Commission Friday after his arrest. "We were in the process of suggesting that he may do better working in the computer field, which is his background," the spokesman said.

County police said Montrose was inside a house on Brentford Drive in the Keene Mill Station subdivision Friday when the 15-year-old youth who lived there returned home about 3:30 p.m.

Police said the youth, whom they would not identify, heard noises in a bedroom and went to a neighbor's house to call police. The youth also let the air out of one of the tires of Montrose's 1978 Datsun, according to police.

As Montrose was leaving the house, police said, he saw Fairfax County police officer Roger Sims responding to the burglary call. After the chase, Montrose was treated at Fairfax Hospital for cuts and bruises and was released to the custody of police.

Sims, asked yesterday if he had said anything to the youth who triggered the arrest, replied: "I just told him he's a very intelligent child."