Garbage in, garbage out. That's what computer programers say when something goes wrong in a computer operation.

Now, however, they may be saying it when things go right.Garbage is the basis of a new electronic bulletin board sponsored by the Chicago Board of Trade. The CBT Recyclables Exchange provides a cyberspace marketplace in which to buy and sell recycled glass, plastic and paper.

But some industry analysts continue to have reservations about the new marketplace.

"We will have to take a wait-and-see attitude if this is a good way of trading scrap materials," said Robert Garino, director of commodities at the Institute of Scrap Recycling Industries, in Washington. His members are mostly recyclers of industrial materials.

"So far, paper is generating more interest than anything else right now," said Barry Kaplan, a consultant who helped design the system. "We have a narrow range of plastics now, but we're finding people who want to do all types of weird plastics that we had never even heard of. It's all very exciting."

The recyclables exchange is the "son of" CBT On-Line, a bulletin board for agricultural commodities, Mr. Kaplan said.

The concept is for bids and offers for such materials as paper, glass containers and plastic bottles to be flashed to industry participants over an electronic bulletin board at the exchange. Prices would be for current delivery, without the time lags built into conventional futures contracts.

A variety of forms of material, weights, and level of contamination can be detailed on the board. For example, if you're selling paper, you can post a listing for corrugated containers, and then provide information on moisture content, dye content, etc.

Establishment of cash market contracts, based on consumer-generated scrap materials like plastics will "help legitimize the industry," said Raymond G. Saba, director of the Center for Plastics Recycling Research at Rutgers University in Piscataway, N.J. It also should spur investor interest in recycling projects, he added.

The concept for such a contract was scoped out a few years ago when post- consumer recycling programs took a quantum leap.

"Cities were offering recyclers good deals on carting off the stuff to bottle and can manufacturers," Mr. Saba said. "In fact, New York City was literally giving it away."

That growth required a formalized contract to help minimize any change of supply disruption and guarantee uniform quality of product, he said.

Plastics reclamation is one of the strongest growing markets for recyclers, according to Dennis M. Sabourin, vice president, post-consumer plastic recycling division of Wellman Inc., Shrewsbury, N.J., one of the nation's largest users of recycled polyethylene terephthalate, PET. Wellman was one of CBT's On-Line system beta-testers.

American plastic producers recycled about 750,000 pounds of plastic waste in 1994, up from 600,000 in 1993, according to industry data. Much of the recycled material was PET, a higher-value engineering grade resin used in making polyester and nylon staple and fiber.

So far, Mr. Kaplan said, no one has completed a transaction on the exchange, which opened for business last Tuesday.

If you're interested in joining the exchange, it will cost you $1,000 a year. Your phone bills for the service are estimated at $200 to $300 a year, according to CBT, depending on how often you use the system.

In the event of dispute arising from an exchange transaction, participants can also use a CBT-created system of quality verification and arbitration.

Interested parties with the proper computer equipment can take a free look at the exchange. Call the CBT Electronic Trading Control Center at (312) 347-5600.