A 50-year-old Washington firm called Rufus S. Lusk & Sons Inc. makes a handsome profit from selling information that is available free to the public.

Lusk compiles its weekly and annual reports on property sales here from the public records on file in the offices of recorders and deeds. The Lusk reports -- listing seller, buyer, address and price -- are "among the most valuable tools of any real estate professional," said one broker.

There were more than 50,000 residential transactions last year here, and Lusk made information about all of them readily available to its nearly 2,000 subscribers. Clients include brokers, appraisers, mortgage bankers, banks, savings institutions, private investors and libraries. Lusk reports are heavily read in the area libraries that stock them, including the District and Montgomery and Prince George's counties.

Separate Lusk reports are filed for each jurisdiction. The Fairfax County report is the area's largest, and last year listed more than 18,000 transactions.

Lusk publications top the reading lists of many real estate professionals, but circulation is relatively small because the service costs several hundreds of dollars, depending on the numbers of directories requested.

Agents read Lusk and compare prices in the annual listings to see what houses are selling for -- and now to determine market values.

Agents say they depend on Lusk for accurate, relatively fast information. The weekly reports, culled by Lusk correspondents from courthouse records, are usually transmitted, compiled and published within four weeks of the transaction in question.

Lusk also records transactions in Baltimore and for much of Maryland and Virginia -- and sometimes as far away as New Orleans.

Founded in 1930 by the late Rufus S. Lusk, the business was expanded considerably after World War II when Rufus Jr. returned from Navy service. The firm moved in the 1950s to Jefferson Place NW, where it still occupies the lower two stories of a former town house.

Last year Rufus III joined the firm and Lusk pere et fils run the business with James H. Molley Jr. a vice president, and Anna L. Stuermann, secretary-treasurer, who has been with the firm since its inception. There are about 60 people on the payroll.

In addition to weekly summaries of transactions and annual compilations of property exchanges, Lusk also publishes reports, statistical studies and zoning maps. The recently updated D.C. zoning map, for example, sells for $1.45.

The transaction reports are the staple of the business, which has used a computer service since 1966.

"Not unjustifiably, we pride ourselves on accuracy," said Rufus Jr. "Some of the records from which we get information are difficult to dig out.

"If our reports is inaccurate, it's usually because the record was. Then, when errors are discovered by those who read our reports, the information is passed on so the recorded deeds can be changed. Nobody's perfect, as they say, but we do try our best to be accurate."