A federal jury concluded today that chemicals dumped at a W.R. Grace & Co. packaging plant polluted two suburban wells in Woburn, setting up a crucial judicial test of whether the toxic waste caused leukemia deaths and other illnesses.

After eight days' deliberation, the jury also rejected evidence that a tannery once owned by the Chicago-based Beatrice Foods Co. contributed to drinking-water contamination in Woburn, a blue-collar town 12 miles north of here.

The case, expected to chart new ground in toxic-waste law, was filed in U.S. District Court against both companies two years ago by the families of seven children and one adult afflicted with leukemia. Five of the children have died.

Jan R. Schlichtmann, the families' lead attorney, today expressed disappointment with the split verdict but said he was "heartened . . . that the jury has held a major corporation responsible for polluting these wells."

"The point is the jury has said this case must continue . . . and that Grace must be held responsible for what it did in Woburn, and that's a very powerful message," Schlichtmann said.

Lawyer Michael B. Keating, representing Grace, said he was confident that his client "will be vindicated" in the trial's second stage, which begins Sept. 15. The families will try to show that exposure to the pollutants dumped at the Grace site caused the deaths and illnesses in Woburn. If the jury agrees, it will determine monetary damages in the trial's third phase.

Expert witnesses for the families are expected to testify that they have direct evidence indicating that the chemicals -- trichloroethylene and perchlorethylene -- damaged the victims' immune systems and lowered their resistance to diseases such as leukemia. Other cases involving toxic wastes have relied on circumstantial statistical evidence of disease among people exposed to the chemicals.

The New York-based Grace Co. owns a food-packaging machine plant about a half-mile northeast of one of the two contaminated wells on Woburn's east side. Beatrice once owned what is now the John J. Riley Co. tannery, about 600 yards across the Aberjona River from a second polluted well.

Families previously reached a $1.2 million settlement with the Unifirst Co., whose dry-cleaning plant is between the Grace facility and the wells. Much of that money has been used to underwrite their lawsuit against Grace and Beatrice.

Anne Anderson, whose son died of leukemia at age 12, said after today's verdict, "Corporate America has been put on notice that no longer can they dispose of their hazardous waste and expect people to accept that."

During five months of testimony, the jurors heard conflicting evidence from 21 hydrologists on the key question of whether the chemicals could have flowed in the ground water from the company sites to the municipal wells.

Grace and Beatrice tried to show that the contamination originated upstream, in Woburn's industrial valley, or from other dumping in the Aberjona River. Grace employes testified that they dumped solvents behind the Woburn plant, but experts testified that those chemicals would not have flowed to the wells.

An expert for Beatrice said that chemicals found at the tannery site were deposited after the contaminated wells were closed.

Beatrice's lawyer, Jerome Facher, also argued that the tannery was on land "flat as a billiard table," making it impossible for ground water to flow to the city wells. Beatrice bought the tannery in 1978 and assumed its environmental liabilities. "It's a complete vindication" for the tannery, Facher said.

Facher and Keating argued that it was unreasonable to expect the companies to know that chemicals dumped on their land would drain into the drinking water supply.