A year ago, officials at Americans for the Arts got a call from lawyers representing Ruth Lilly, an heir to the family pharmaceutical fortune. She had been a supporter of the Washington-based advocacy group for 25 years, but her lawyers said she wanted to do more.

Yesterday the organization told its members that Lilly had decided to do much, much more. She is giving the group $120 million, to be distributed over the next 30 years.

Americans for the Arts, a relatively small organization dedicated to educating decision makers and citizens about the value of the arts, is now trying to figure out how it will spend the money.

"I was filled with gratitude and humbled at the same time," said Robert L. Lynch, the group's president. "She has put a great deal of trust in us. She knew it would have a really long-term benefit and help the cause of bringing access to the arts for everyone."

In the Washington philanthropy sweepstakes, the Lilly gift eclipses the $100 million given to the Kennedy Center only 11 days ago by the Catherine B. Reynolds Foundation. It is the largest gift ever given to any of the cadre of small lobbying organizations that help arts agencies, administrators and museums negotiate the sometimes turbulent waters of legislation and philanthropy.

The total amount is roughly equivalent to the annual budget of the largest public supporter of the arts, the National Endowment for the Arts, which received $115 million from Congress in fiscal 2002.

The donation to the arts group is the second extraordinary and unconventional gift from Lilly in recent weeks. Last month she gave $100 million to Poetry Magazine, a small but prestigious journal that has published the work of William Butler Yeats, W.H. Auden and Dylan Thomas. Lilly's own poetry had been rejected by the magazine.

Lilly, 87, lives in Indianapolis. She is the last surviving great-grandchild of Col. Eli Lilly, who founded the drug company that bears his name. For most of her life, she has been a reclusive semi-invalid, suffering from depression. According to published reports, she requires a wheelchair because of a spine problem, but her mental condition has been improved in recent years thanks to Prozac, one of Eli Lilly's most famous products.

Over the years, Ruth Lilly has been a generous supporter of the arts and education. At her death, according to the Indianapolis Star, half of her estimated $1 billion estate will go to the arts and various charities. Much of the donation to Americans for the Arts is Eli Lilly stock from a trust, and the payments will continue in the event of her death. In 1981 a guardian was appointed by the court to monitor her financial dealings.

Lynch said he wasn't sure how Lilly became interested in the organization because the people who first dealt with her are deceased. "We have been involved with Ruth Lilly for over 25 years. She has, through her lawyers, talked about a larger gift on several occasions," Lynch said. In the past, her contributions ranged from $10,000 to $15,000.

"While the actual timing and value of the distributions . . . will depend on a number of factors, the aggregate value of the distributions is expected to exceed $120 million," Lynch told his members yesterday.

Americans for the Arts was set up to provide research and information to encourage elected officials and the public to support the arts. Earlier this year, for example, the group released a national survey that concluded that nonprofit arts institutions generate $134 billion in economic activity each year. Each spring, the group holds a day of lobbying, called Arts Advocacy Day, which pairs celebrities with arts administrators. These teams fan out across Capitol Hill, calling on members of Congress and other key officials to talk about the importance of the arts. It has enlisted Garth Brooks, Harry Belafonte, Marlo Thomas, Alec Baldwin, Debbie Allen and Anita Baker, among many others.

The group was founded in 1960, and the current organization is the result of a merger six years ago of the National Assembly of Local Arts Agencies and the American Council of the Arts. It has a staff of 38 and offices in Washington and New York. Its headquarters is a 9,000-square-foot suite on Vermont Avenue NW at K Street, with a view of the White House grounds.

The group's current annual budget is about $8 million. Lynch said yesterday that he is establishing an investment committee and hiring a consultant to help manage the Lilly funds. The group has about 2,000 dues-paying members, including arts commissioners, philanthropists, mayors and institutions.

The Lilly money, Lynch said, will help provide more leadership training, scholarships and research. He said that after the membership responds to a survey about the organization's needs, the group will develop a strategic plan.

Pharmaceutical heir Ruth Lilly in an undated photo.