The phenomenal turnover in houses in the Washington region, where 100,000 households move every year, helped give rise to the use here of "lock boxes" to give real estate agents easier access to houses they are trying to sell.

The 7,000-member Northern Virginia Board of Realtors began using them several years ago in their highly transitory area on houses sold through the board's multiple listing service. The strong presence of military and military-related people who are transferred regularly make Northern Virginia a big resale area. Additionally, private businesses contribute to the game of "musical residences," shuffling middle-executives almost as frequently as their military counterparts.

The increase in two-income households has also helped spur the market for houses and, as a result, many of those houses are unoccupied from 8 a.m. to 6 p.m. while the occupants are off making money to meet mortgage payments.

When houses are listed for sale, realtors in Northern Virginia often ask to install lock boxes so they may bring prospective buyers in.

The lock box is attached to the door of a house and a key to the house is kept inside. The box is opened by a common key, distributed in Northern Virginia to licensed agents who pay $30 for the priviledge and who agree to certain security measures.

The board's president, Donald R. Childress, says the easy access lock boxes afford is a fundamental tool for selling houses.

"It broadens the possible exposure of the house for sale and makes it possible for the house to be shown without forcing the owner or occupant to be constantly sitting at home waiting for a salesperson to bring in a client," Childress said.

He said that the alternatives are to have a house open by "appointment only," or to restrict the availability to one agent who has a key and can pass it around, at some inconvenience, to other agents who have interested buyers.

"We are convinced that we can sell properties quicker by having the listed dwellings available to thousands of agents and their clients at any one time," Childress added.

Generally, the lock box system had been working well and without much criticism until a few months ago. Then a score of listing owners of properties began complaining that their houses had been entered without visible sign and burlged of valuables.

At first, the realtors board contended there was no evidence that lock box entry was used. But two months ago a 15-year-old son (in a household whose house was listed and fixed with a lock box) went home after school and found a burglar at work.

The lad reacted intelligently and notified police, who apprehended a suspect and eventually turned up a cache of valuables taken from Northern Virginia homes.

The suspect is a licensed real estate agent with legitimate access to a lock box key. The Northern Virginia Board of Realtors says the robberies were the result of "one bad apple in the barrel," and are not an indication that they system itself is unworkable.

(The youth whose superb performance resulted in the arrest has chosen to remain anonymous. The realtors board presented him with $2,000, and said it was hoped that he would use the money for college.)

The board hopes that the arrest ends the period of problems with lock box entries. But, understandably, there remains some resistance among homeowners to having lock boxes stand between strangers and their valuable possessions. Childress and other brokers are urging clients to remove valuable silver from their houses and to lock up valuables that can be easily picked up. Owners still have to assume any risk when their houses are lock-boxed.

Owners also are asked to sign agreements stating that they understand that appraisers, inspectors and other real estate industry people may have access as well as agents. The agreement also states that agents are not responsible for vandalism, theft or "damage of any nature whatsoever to the property."

Fairfax County police, who have investigated a number of complaints involving lock-box-house thefts in recent months, consider the use of this real estate marketing device a hazard to home security.

"Some people now in real estate are known to have previously been involved in criminal activity," said police Maj. Richard H. Lester. "That's like letting a fox in a hen house. I know it's a convenience for agents and for selling, but it does have some drawbacks. We have suggested that the master keys be changed more frequently."

Arthur W. Kogstadt, executive vice president of the Northern Virginia Board of Realtors, said that controls of keys have been tightened and master keys will be changed more frequently. But he and Childress say realtors want to continue with lock boxes because of the convenience.

In Montgomery County, lock boxes have been used for several decades without incident, said Hans Nestler, executive vice president of the county's Board of Realtors. Most often the lock boxes are used on unoccupied houses, where the threat of burglary is minimal, he said.

The Washington Board of Realtors, which only earlier this year adopted a multiple listing service for the District, does not use lock boxes. One veteran realtor suggested that few of the really expensive houses are likely to have lock boxes because the "appointment only" system is often used for prestige and security reasons.

In Prince George's County, the Board of Realtors has never used lock boxes on its multiple listing houses, said board executive Paul I. Fowler. But he said individual brokers, especially the larger firms, use them for their own listings and their own agents. Fowler said he has been discouraging the use of lock boxes on multiple listing houses - "over the objections of some members. Now they are agreeing with me."

Since the reporting of lock-box-home burglaries in Southern California and a few other parts of the nation, the National Association of Realtors in Chicago has issued an "advisory" urging local boards to disassociate themselves from the procurement, sale and issuance of lock boxes. The association has also urged more action on security of houses listed with members and is studying measures that could be recommended.

Chesley Smith, director of membership politcy and procedures for the national association, said that lock boxes have been used in many areas without any problems and are a tool favored by members.

But as Childress of the Northern Virginia board reminded all principal brokers and sales managers: "Use of the lock box is not mandatory. Homeowners may, if they prefer, list their homes 'by appointment only.' Disadvantages of the latter choice should be noted by explaining to them the difficulty salespersons may have in showing the home if they are not able to contact the owner or the listing agent."

Additionally, the Nothern Virginia Board has amended its rules recently to make the charge for a second key (if the first is lost) $100 after a written report is filed by the losing agent.

If the realtors seem to be stubborn in their devotion to the lock box, it probably is simply because they are convinced that they can police their own membership and use the lock box tool with discretion, while warning clients to take all possible precautions.

"Our members are disturbed that one of their own has been apprehended in the lock box theft situation. . But they also are more likely to run tighter ships in their own offices and try to bring the situation back to the days before January 1978, when there really was no lock box security problem," Childress said.

In a non-related program, he said, the board recently worked out a "neighborhood watch" program with the National Sheriffs Association to reduce theft and vandalism in any area where residents want to participate. The idea is to have residents be alert to anything unusual when a neighbor is away.