The Better Government Association, a local group of investigators and lawyers with a long string of successes in teaming up with the news media to expose corruption in government, plans to open a Washington, D.C., office and offer its services to national news organizations under a $2.5 million, five-year foundation grant.

William T. Kirby, attorney for the huge new John D. and Catherine T. MacArthur Foundation, confirmed today that the grant will be made to the 55-year old muckracking organization, which in recent years helped The Chicago Tribune win two Pulitzer prizes for investigations into vote fraud and ambulance services.

"We hope that the BGA will turn up some serious government abuses that no other organization is set up to uncover," said J. Roderick MacArthur, son of the billionaire who established the foundation. "But we're really supporting whatever the BGA wants to do because we believe it has done a hell of a good job in Illinois."

J. Terrence Brunner, executive director of the BGA, said, "The grant will give us new capabilities to do what we're doing, to do more of it and to work with the national news media."

Brunner, 41, a former assistant U.S. attorney in Minneapolis, said the grant will enable the group to double, approximately, its $500,000 annual budget in each of the next five years. Almost all of the additional money is to go to expand the staff and operate the Washington office, he said.

The BGA has 18 full-time employes, including four lawyers, who work on investigations in Chicago and Spring-field, the state capital.

BGA investigators have worked with all major Chicago newspapers and several television and radio stations on investigations ranging from the secret financial dealings of Chicago politicians to abortion clinics.

The organization's work has led to several indictments and convictions of officials in Illinois -- the most notable being Alderman Thomas E. Keane, whose political power in Chicago was considered second only to that of the late Mayor Richard J. Daley. Keane was convicted of mail fraud in connection with secret land deals in 1973, and only recently was paroled from federal prison.

The BGA's best-known activity locally was an investigation into tavern shakedowns by public employes in 1977-78. Working with The Chicago Sun-Times, its investigators operated a tavern aptly named the Mirage on the north side of Chicago for several weeks to gather first-hand evidence of shakedowns. Several indictments resulted from the investigation.

Only once has the BGA done a national investigation. In 1975-76 its investigators worked with CBS-TV reporters on an investigation of fraud in the federally financed Medicaid program. That investigation prompted passage of the Medicaid Fraud Act, and drew an admission from Health, Education and Welfare Secretary Joseph A. Califano Jr. that the fraud totaled $3 billion a year.

Kirby, who is a director of the MacArthur Foundation as well as its lawyer, said the foundation's five acting directors have agreed unanimously to provide the money, but still must formally vote a resolution. "We definitely will finalize it in early March," he said.

The BGA grant will be the third approved by the foundation, which began operations late last year after taking possession of 100 percent of the stock of Chicago-based Bankers Life and Casualty Co. worth an estimated $750 million.

Bankers Life was the principal asset of John D. MacArthur, who set up the foundation in his will. MacArthur died in January 1978 at age 80.