Researchers planned a second experiment yesterday to gauge effectiveness of oil-eating microbes on an oil spill at sea after the first test went awry and yielded no data, according to scientists circling the vicinity of the supertanker Mega Borg in the Gulf of Mexico.

In an attempt to test whether the microbes would digest the oil and turn it into harmless byproducts, the bacteria were sprayed Friday onto part of the oil slick off the coast of Galveston, Tex., by University of Texas oceanographer Carl Oppenheimer, who had collected the creatures from dozens of oily locales worldwide.

But the portion of the oil slick that was sprayed was not marked by a buoy or enclosed with floating booms. Researchers aboard a Texas A&M University research vessel, which arrived at the experiment site early yesterday to sample the water, said they had no way of knowing whether they were sampling the water doused by Oppenheimer, because wind and currents could have carried the bacteria-treated water miles away.

"Right now, the probability of determining whether the bacteria had any effect is zero," said Terry Wade, chief scientist with the Geochemical and Environmental Group at Texas A&M, in a telephone interview from the research vessel Gyre. "We were under the impression that they'd be using some sort of enclosures."

Without enclosures or the deployment of some kind of buoy, Wade said it would be impossible to know where the bacteria-doused section of the oil spill was, since a slick is constantly changing direction and shape, a victim of weather and sea conditions. The matter is further complicated by the fact that oil degrades naturally and that oil-eating bacteria other than Oppenheimer's occur naturally in the gulf.

Oppenheimer and his team said it was impossible to deploy enclosures from the 40-foot Coast Guard vessel they were aboard Friday, Wade said.

In the experiment scheduled yesterday, Wade said two enclosures would be placed upon the oil slick and bacteria added to one, but not the other. Water from two enclosures would then be tested over the next 24 hours for signs that the bacteria were doing their job. Preliminary results are expected today.

The effectiveness of microbes on oil has been well established for years. But many researchers question whether the bacteria will be able to rapidly break down the oil contained in a large spill at sea.

The Coast Guard said yesterday that 12,000 to 40,000 gallons of oil that were part of more than 4 million gallons spilled by the Mega Borg, which caught fire after an explosion June 8, remain in the water about 25 miles from Galveston.