Engineers will let billions of hungry microbes loose on tar-like waste left by an old gas works in hopes of cleaning up the mess in three years, not the 1,000 nature would need if left to itself.

The stuff is on a site called Blekholmstorget, in the shadow of the gold- tipped brick tower of Stockholm City Hall, where an apartment and office complex is planned.Biotreatment of Cardiff, Wales, developed the use of microbes to break down ground pollution, and the technique has been employed in Britain and West Germany. This is the first use in a major cleanup that does not involve hauling away the contaminated soil.

Hans Kronberg, chief engineer, said the goal is to purge potentially cancer-causing grit from about 1.2 acres on the bank of Lake Klara, a channel in Stockholm's interlocking chain of lakes. Dozens of pleasure boats now dock there.

A gas and coke factory was built on the site in 1852 and operated for about 70 years. Most recently, the site was a parking lot for the nearby central railway station.

Mr. Kronberg said it took scientists nine months to identify specific microbes that attackthe pollutants involved. Bacteria were taken from the site and fed oxygen and nutrients to help them multiply.

When the project begins in May, the plan is to filter microbe-enriched water through the soil, which is soaked with creosote, breaking down the tar- like waste.

Grids of pipes spaced a few yards apart cover the site. Enriched water will seep from one set of pipes and be collected by another 10 feet below. The water will be pumped to a container for analysis, treated with nutrients and recycled into the pipes.

"As of this moment, we can't say if it will work or not," Viveka Ehd- Falander, city inspector, said. "We have never done this in Sweden before."

Stockholm officials and federal environmental authorities see the biological cleanup as a pilot project for opening hundreds of polluted acres in the city to development.

Mr. Kronberg, who works for the Skanska construction company, said the creosote at Blekholmstorget was not a problem while it was buried under the parking lot asphalt, but would become a threat under a major housing development.