Prince George's County real estate agents are moving to reassure sellers about the safety of their houses when they are being shown to prospective buyers by replacing traditional key- and combination-operated lock boxes with a system that records on film who was the last agent to enter a home.

"We've had theft problems with the old lock-box system," said Paul L. Fowler, executive vice president of Prince George's 4,000-member Board of Realtors. In one case, the furniture from several model homes was cleaned out by someone using a lock box to gain entry, Fowler said.

"The advantage of this system is that, every time a particular user inserts the key, it is recorded chronologically on the film so we can determine who was in the house last," he said.

About 5,100 boxes made by an Oregon firm, Multacc Service Corp., are being used by about 1,000 agents in Prince George's, according to Multacc manager Harry Renninger.

Under the new system, an agent inserts a key into a Multacc box and then taps in an individual five-digit identification number. A correct ID number opens the box and records the agent's number on film for future reference.

About 40 realty boards in California, Texas, Florida and Las Vegas have authorized the Multacc system, and some agents in Montgomery County are considering making the switch, Renninger said.

The use for lock boxes first became widespread in the 1950s as a more convenient way to make for-sale homes readily accessible to numerous agents and their clients.

Traditionally, house keys were kept in the boxes, which usually were hung on door knobs or front porches. Realtors who wanted to see or show a home opened the lock box by using a master key that also could be used to open every other house's lock box.

In Prince George's County, many agents still use a box with a combination lock, but Fowler said the combination "is very often the same three letters. Everybody knows what they are, and that blows the security of that system."

Lock boxes in their various forms save real estate agents from having to arrange to get the house keys from the owners or to drive across town to the listing agent's office to get a set of keys.

But until the new electronic systems came on the market, if someone spilled coffee on a plush living room rug, tracked mud or pilfered valuables, homeowners had no way of telling which visitors were to blame. Frequently, under the old lock-box systems, agents were issued six or 10 master keys and there was little or no effort at accountability.

"Obviously, the keys could be lost or get into the wrong hands and sometimes did," said Peter Jacula, director of multiple listings for Northern Virginia's Board of Realtors. More than 11,000 agents in Fairfax, Arlington, Prince William, Fauquier and Loudoun counties and Alexandria use an improved lock-box system to show houses, he said.

In one highly publicized case in 1978, a Century 21 agent pleaded guilty to thefts in five Northern Virginia homes after gaining admittance by using keys in a lock box. Police alleged that the man had stolen $200,000 to $300,000 worth of furs, jewelry and silverware from 100 homes with lock boxes that he had entered during a four-month period.

Area Realtors variously describe the 1978 case as "unfortunate" and an "isolated incident," but they say Washington-area realty boards go to great expense to make security a priority.

In Northern Virginia, lock-box keys currently used by agents will become obsolete next Nov. 1 when the six-county area reissues master keys, Jacula said. Agents will get "one key and one key only, and they are not to let anyone else use it. If they lose it, they forfeit their $100 deposit and must pay $200 for a new key," Jacula said.

Jacula said Realtors in Northern Virginia considered all new electronic systems as well as the Multacc system, but decided that theft and security problems were not great enough to spend the estimated $1.5 million it would take to convert to a system similar to Multacc's.

In contrast, Jacula said replacing keys in the Northern Virginia system will cost about $150,000, partly because the lids of the lock boxes also must be changed. The last time the system was rekeyed was in 1983, he said.

Realtors in the District of Columbia started their first lock-box system ever on Jan. 21, according to David Strachan of the D.C. Board of Realtors.

In the District's Supra patented key system -- which is the same that Northern Virginia and Montgomery agents use -- each agent buys one master key that has an identification number and is accountable for it.

"We decided to go with the old tried and true lock box method because the technology is moving so quickly that the new electronic systems might be obsolete in a year," Strachan said.

William D. North, senior vice president and general counsel for the 712,000-member National Association of Realtors, said he did "not see any very strong industrywide trend in the direction of the Multacc or other electronic systems." North said there is "no great epidemic" of thefts related to older, key-operated lock boxes. "We'd know, because we have to protect the state boards who are sued by homeowners," he added.

North said, "One of the problems we have is lock-box sellers using fear as a major sales tool . . . The Supra lock box continues to offer excellent security," but he said local realty boards must control who gets the keys and report their losses immediately.