The 1990s will be the decade when ''purpose'' investing comes into its own. The social activists among the baby boomers have reached the age of saving and investing -- and they'll support their causes financially.

Here are a number of ways that you can put your money where your mouth is:

Socially conscious investing is the main event. After a slow start in the late 1970s, white-hat mutual funds now control in excess of $500 billion, according to the Social Investment Forum in Minneapolis.

Some mutual funds deliberately seek out companies thought to be socially benign -- for example, firms that develop alternative energy sources or new methods of pollution control.

Other funds merely avoid the black hats, usually defined as companies with ties to South Africa (Nelson Mandela wants the boycott continued), tobacco stocks, defense contractors, utilities dependent on nuclear power, major polluters and companies with a history of employment discrimination or bad labor relations.

Be sure you read the prospectus. Just because a fund has ''environment'' in its name doesn't mean it really looks for progressive companies. The following stock and bond funds belong to the Social Investment Forum: the Calvert Group in Maryland (800-368-2748); Pax World in Washington, D.C. (800-767-1729); Parnassus in California (800-999-3505); and New Alternatives in New York (516-466-0808). For money-market funds, try Calvert, Working Assets (800-699-7725) or the South Shore Bank Fund (800-669-7725).

A bright idea that's likely to be copied by many other organizations is ''affinity'' long-distance telephone service, pioneered by Working Assets in San Francisco (800-522-7759). When you sign up through Working Assets, you become a customer of US Sprint, paying exactly the same rates as any other customer.

Sprint returns around 2 percent of the value of your calls to the organization. One percent of the value of your calls goes into a donation pool. It's distributed once a year to nonprofit organizations in four different fields: environment, peace, human rights and hunger. Last year, the group gave out $365,415 to 32 organizations nominated and voted on by its members. Among the leading vote-getters: Greenpeace, National Abortion Rights Action League, National Gay & Lesbian Task Force, the Environmental Defense Fund and Amnesty International.

All telephone proceeds over 1 percent are used to defray Working Assets's costs, says chief executive officer Laura Scher. The service isn't breaking even yet, but Scher hopes that eventually it will be in the black.

Affinity credit cards were also pioneered by Working Assets, followed by the Sierra Club. Every time you use a nonprofit group's Visa or MasterCard, it earns a small fee from the sponsoring bank. At Working Assets, $2 per card goes into the donation pool, plus five cents for each transaction you make. The cost of the card: $20 a year (waived for the first six months), plus 17.5 percent interest on revolving credit.

Working Assets also offers Women's Card. The proceeds go to the card holder's local women's foundation. If there isn't one, the beneficiaries are the Ms. Foundation and the National Network of Women's Funds.

One more idea from Working Assets: an affinity travel agent (800-332-3637). You can book plane tickets, hotels, rental cars and tours through the services of Sigma Travel in San Francisco. Sigma antes up 2 percent of your purchases to Working Assets's general donation pool.

You don't get tax deductions for these kinds of donations, but your contribution doesn't cost you anything either. For social activists, it's a free lunch.