What has two wheels, one seat and can help fight five of this country's fiercest foes: inflation, pollution, fuel depletion, traffic congestion and . . . flab?

Answer: the bicycle. Once considered kid stuff, it is rapidly becoming the adult transportation alternative of the '80s. Since 1972, bicycles have been outselling passenger cars in the United States as increasing numbers of gas-crunched Americans are choosing to pump pedals, not gas.

Commuters who've looked on in envy (or terror) at cyclists whizzing past snarled traffic are welcome to dust off their two-wheelers and pedal along Tuesday with the 1,500 Washingtonians expected to participate in the Earth Day Bike-In.

"The biggest biking event in Washington's history," according to organizer Peter Harnik, will consist of 25-plus convoys of cyclists pedaling downtown from locations all over the city and suburbs.

"Convoys will begin from as far away as Annandale and Hyattsville," says "bike fanatic" Harnik, one of Washington's estimated 30,000 regular bicycle commuters. "As they get closer to the city, they'll be joined by other cyclists who'll swell the ranks."

The cycle convoys will converge at Lafayette Park at 8:30 a.m., where musicians Mike Seeger and Alice Gerrard will entertain with bluegrass, folk music and "bike-song." Dennis Christopher, star of "Breaking Away," will be among celebrities and politicians who will address the crowd.

Bike-In balloons, T-shirts and posters will be available, a healthy "biker's breakfast" will be on sale and an award will be given to the federal agency with the most employes riding bikes that day.

"We want to call attention to biking as an energy-efficient, non-polluting, quiet, convenient, fast method of commuting," says Harnik. "And we're demanding, in a nice way, that bicycles be given a higher priority in terms of transportation planning for the '80s."

Rather than treating bicycles as "second- or third-class vehicles," Harnik would like to see "separate bike lanes exclusively for cyclists, better bicycle access along bridges, more bicycle racks and shower facilities for commuting cyclists.

"Washington is way behind other cities such as Davis (Calif.), Seattle and New York in terms of city services for bicyclists. Here we are in the nation's capital, but instead of providing the leadership we should, we are far behind."

Harnik hopes a heavy turnout "will demonstrate to city leaders the strong commitment of Washington cyclists. This event will show there are many thousands of bicyclists willing to take cycling seriously."

The Bike-In is one of several city-wide events scheduled to celebrate the tenth anniversary of Earth Day, first observed April 22, 1970, to launch the environmental movement. Sponsoring organizations include six area bicycle clubs, the U.S. Environmental Protection Agency and the U.S. Department of Energy.

"All signs suggest that 1980 is a boom year for bicycling," says Department of Transportation bicycle program coordinator Bill Wilkinson, whose agency is in the process of releasing to the president and Congress a report on bicycle transportation for energy conservation.

"People are no longer reluctant to be seen commuting by bicycle," says veteran cyclist Wilkinson, who, 10 years ago, attracted stares when he carried his safety helmet into the office. "We'll be documenting real gains to be made by bike use. I think we're going to take a more activist role."

On Tuesday, rain or shine, prospective pedal-pushers are urged to arrive at the convoy starting sport closest to their homes 15 minutes before scheduled departure time.

"Every bicyclist is welcome," stresses Harnik. "The convoys will be led by experienced cyclists who will ride at a pace recreational cyclists will be able to keep up with. Any bike in average condition should be fine.

"The convoys are designed to be as safe as possible, with police taking special precautions to ensure bikers' safety along the routes. There will be at least one person in each convoy with a bike pump and repair kit."

Bikers are advised to:

Wear bright clothing.

Carry possessions (including a change of clothes, if desired) in a backpack or saddlebag.

Wear a helmet if they have one and a poncho if it rains.

Have a bell or horn on the bike.

Ask their convoy leader where and when to meet for the "Bike-Out" of cyclists who want to ride home in a group.

Call 265-4317 (the Washington Area Bicyclist Association) for more information.

"Bicycling offers advantages both to the individual and society," notes Harnik. "Not only does it save you money and improve you physical condition, but it cuts down on air pollution, congestion and our reliance on imported oil. Best of all, it's fun!"