Carol Laskin was incredulous when she realized her 6-month-old baby was losing weight. She had followed her doctor's advice and breast-fed Benjamin until his fourth month, then weaned him to the formula Neo-Mull-Soy. But within a few weeks Benjamin refused to take his bottle. His skin felt doughy. And he stopped growing.

The baby's pediatrician tested Benjamin's blood and told the Laskins of Northwest Washington to take him straight to Children's Hospital, where a kidney specialist told them the baby had a rare and sometimes deadly kidney disease called Bartter's syndrome.

To humor the baby's grandmother, the specialist agreed on the third day of Benjamin's hospitalization to switch formulas. To the physician's astonishment, the baby gulped down the new food and recovered so quickly that the Laskin were able to take him home that afternoon.

Medical authorities soon discovered that Benjamin was one of at least 118 babies who refused their food and stopped growing properly after being put on Neo-Mull-Soy or a more specialized product called Cho-Free. Some babies had seizures. One stopped breathing. Two infants died, although doctors still don't know whether the formulas were to blame for their deaths.

Parents and doctors still worry, weeks and in some cases months later, that the surviving children may suffer permanent brain damage or learning disabilities.

Tests showed that the two formulas manufatured by Syntex Corp. from mid-1978 until August of this year lacked at least one vital nutrient that left the affected children malnourished during the critical months months when brain growth is most rapid.

Twenty thousand American babies were on the formulas. Some became ill as early as October 1978, but no one connected the cases of apparent kidney disease to a problem with the Syntex formulas until last July. Syntex recalled Neo-Mull-Soy and Cho-Free August, although the formulas were still available in some stores in Washington and elsewhere in late October.

The Food and Drug Administration, which at first refused to give the recall top priority despite a recommendation from its own experts, acknowledges it failed to monitor the firm's recall as closely as it should have.

Syntex it plans to put the formulas back on the market as soon as the FDA is satisfied that chloride, the one vital ingredients known to have been lacking, has been restored in sufficient quantity.

But some medical experts doubt the lack of chloride alone explains the babies' illness.

Dr. Jose Salcedo, the kidney specialist who treated Benjamin Laskin, said some of the symptons he saw could not be caused by a chloride deficiency and may have been the result of some still unidentified ingredient.

Rep. Albert Gore (D-Tenn.), whose child was given the formula, held hearings on the strange outbreak last month and called for tighter government regulation of the formula industry.

The incident also has sparked a dozen lawsuits against the company, including a $2 billion class action claim filed in U.S. District Court in Memphis.

Benjamin Laskin's illness in May was the first Washington-area case of Bartter's syndrome in a Neo-Mull-Soy baby, according to Dr. Salcedo. The kidney specialist said he thought it curious at the time but had no reason to believe then that it was part of an epidemic.

In late July, however, a Memphis pediatrician alerted the federal Center for Disease Control that he had seen three cases of the rare disease in babies fed Neo-Mull-Soy. A preliminary survey by the CDC quickly uncovered other cases across the country.

Dr. Jose F. Cordero, a CDC investigator, said the babies' blood tests showed abnormally low levels of chloride, sodium and potassium. Some had blood in their urine. And several had extraordinarily high levels of renin and aldosterone, two kidney-related hormones that sometimes cause dangerously high blood pressure.

Of the 118 babies eventually identified by the CDC, nearly all recovered after they were given chloride and a different formula. But Cordero is reviewing the records of two who died. He said one had been ill before receiving Cho-Free and was on it very briefly, and the other did not have the blood abnormalities seen in the other infants.

The problems with the formulas apparently began in spring of 1978 when Syntex removed sodium chloride -- common table salt -- from the two products. Syntex President Paul Freiman said last month the salt was dropped because of the "prevailing climate of opinion. . . that having salt in baby food is bad for children."

A Neo-Mull-Soy advertisement that Syntex ran in medical journals as recently as last June boasted, "It's what's left out that's important." The ironic ad referred to the absence of cow's milk, corn and lead, not to the removal of salt or the inadvertent lack of chloride.

Freiman's suggestion was challenged by Dr. Samuel J. Fomon, professor of pediatrics at the University of Iowa Medical College and a leading authority on nutrition, who said that sodium and chloride are recognized as necessary nutrients and that there has been no general movement to remove them from formula, though they have been taken out of some baby foods intended for older children.

Syntex apparently believed its product contained other sources of the two chemicals, but the company had stopped testing for chloride in late 1977, according to information the firm gave the FDA. When the salt was removed, the chloride content of the formula dropped below critical levels. But according to Syntex officials, no one was aware of that for more than a year.

Throughout late 1978 and the first half of 1979, babies with apparent Bartter's syndrome were turning up in doctors's offices. Many had symptoms like Benjamin Laskin's which pediatricians often call "failure to thrive." A few underwent dangerous kidney biopsies. Others recovered when their doctors changed formulas. Many conditions may have been diagnosed as "colic," which went away with a change in food.

When the Laskin's doctor checked his records, he found other babies in his practice who has grown poorly on Neo-Mull-Soy. "Now we see how many other children were having problems and we were never able to put our finger on it," he said.

Sometimes, babies' symptoms were so subtle that only the mothers knew anything was wrong. Lynn Pilot's 4 1/2-month-old son Bradley began refusing his formula during a family vacation last July. She took him to prominent pediatricians in Cleveland and Detroit, then to her family doctor in Springfield, Va. They told her the baby looked fine.

Pilot wasn't satisfied. "I could just see him disintegrating in front of me," she said. After the baby lost two pounds in two weeks, her pediatrician agreed to admit him to Children's Hospital.

Then Pilot read a newspaper article about Neo-Mull-Soy and inisted on blood tests.

Bradley had the same severe abnormalities as Benjamin Laskin. Even after the formula was changed, he refused to eat and screamed whenever he saw a spoon or nipple.

Lynn Pilot said he improved after several days of intravenous feeding, but was not able to take a full bottle of formula for two months. On her most recent visit to the pediatrician she was told he was again growing well but was "a little slow" on tests of brain development.

Syntex says it spent more than $4 million notifying doctors, drugstores and supermarkets, and destroyed more than two million cans. But in October, General Accounting Office investigators found Neo-Mull-Soy still on sale in five cities around the country. Last month, Gore criticized FDA for not assigning top priority to the recall even after the agency's own Health Hazard Board unanimously voted the formulas constituted a "life-threatening" hazard.

The FDA presently regulates baby formula as a food, which means it has power only to require accurate labeling and to monitor the conditions under which the formula is manufactured, said Wayne Pines, the agency's spokesman. The agency has much more extensive power to ensure the drugs it regulates and both safe and effective. Gore introduced a bill in Congress last month that would increase FDA's authority over baby formulas and require that new products be tested before they are permitted on the market.

But Larry Pilot, the father of Bradley and a former FDA lawyer, argues that the agency has sufficient power now, if it would choose to regulate formula as a drug instead of a food. He said the drug definition has been used in the past for honey, surgical sutures, and intravenous feeding solution. u

The CDC and National Institutes of Health plan to watch the development of Neo-Mull-Soy babies to determine whether the formula caused permanent damage.

Dr. Formon, the nutritionist, said lack of some nutrients, such as vitamin B-12, can lead to retardation, but no such effect had been shown in cases of chloride deprivation.

Other doctors, however, remain concerned.

"You have to think that a child who's been malnourished during some of the more important months of his life. . . is possibly in for it," the Laskins' pediatrician said.

There also is the possibility that another, still unidentified ingredient in the formulas was responsible. Cordero said he cannot explain why only 118 of the thousands of babies fed the formula are known to have developed serious symptoms.

The high levels of the hormones renin and aldosterone in the Pilot and Laskin babies cannot be explained by chloride lack alone, Salcedo said. He believes the formula contained some substance that mimicked another hormone called a prostaglandin, or perhaps a chemical that certain babies were able to convert to prostaglandin. Those infants then developed all the signs of Bartter's syndrome, which is caused by excess prostaglandin.

For Carol Laskin, who is anxiously watching Benjamin's progress, the ordeal is far from over.

"Every morning when I hear his little bell, when he kicks it with his foot, I feel much better," she said. "I think, 'Oh God, we made it through the night.'"