Don't leave home without it.

The popular advertising slogan is about to take on special meaning for Mount Vernon.

The Mount Vernon Ladies' Association has worked out an agreement with the American Express Company and other local businesses that could yield the historic property at least $100,000 for its first endowment fund.

According to the plan, Mount Vernon will receive 3 cents each time an American Express card is used in the Washington area during February, March or April. Hecht's, Woodward & Lothrop and Marriott also will participate in the program.

The money will be the initial corporate contribution for an endowment fund for capital improvements, according to John A. Castellani, association director.

The Ladies' Association purchased George Washington's estate in 1858 and since then has preserved and maintained the 500 acres overlooking the Potomac as a nonprofit institution. They run the estate from admissions fees without government support.

The association is in the midst of a $10-million fund-raising campaign, $4 million of which will be the endowment. So far the fund-raising effort--the first for the association since it purchased the estate--has raised $3.3 million in gifts and pledges, Castellani said.

The venture coincides with the 250th anniversary of Washington's birth on Feb. 22. Castellani said the money will be used primarily to finance a new library and research center on the estate and to update electrical systems, security and fire protection for the 42 existing buildings. The association also hopes to conduct the first comprehensive archeological survey of the estate, Washington's home from 1754 to 1799.

When the 31 women in the association learned of the American Express proposal last November, Castellani said, there was some hesitation at first. "The association has never involved itself in any commercial activity," he said. "It has been the policy not to endorse the use of Mount Vernon in advertising that was exploitation of a national shrine. But the nature of this proposal was quite different. It was designed to have them endorse us."

"The association is extremely grateful for American Express' interest in Mount Vernon," he said.

Mount Vernon is not the first group American Express has chosen to help, said Bill Rolle, a company spokesman in Washington. In similar arrangements, the company last year raised $47,000 for the Museum of Contemporary Art in Los Angeles and $102,000 for the Dallas Ballet.

"The challenge," Rolle said, "was how to help charities and nonprofit groups that need help in an unusual way and still help the American Express Company."

In Northern Virginia, the offer applies to cards used from Feb. 1 to April 30 in Arlington, Fairfax, Loudoun and Prince William counties as well as Alexandria, Falls Church, Fairfax City, Manassas, Manassas Park, Reston and Herndon.

In addition to the 3 cents per charge transaction, Mount Vernon will receive 5 cents for every purchase of American Express Travelers Cheques and $2 for each area resident whose application for a new American Express card is approved during the campaign.

The other companies participating in the program have their own formulas for contributions. Hecht's plans to donate 6 cents for American Express transactions in its area stores from Feb. 1 to Feb. 22. Woodward & Lothrop will donate 3 cents for each transaction from Feb. 22 to March 30. Marriott will make a $1 contribution for each card transaction in its gift shops and at its Sunday brunch from Feb. 1 to April 30.