Every Wednesday, between 25 and 40 children show up at the John F. Kennedy Institute here to find out how much lead they have in their blood. All of them have been poisoned, most by swallowing lead paint dust or chips.

The doctors and technicians at the hospital for handicapped children measure the blood, but there is little more they can do besides rather optimistically suggesting that the young children not put things in their mouths, or that the parents find somewhere else to live.

Lead poisoning, most of it attributed to lead paint, is still a serious problem in Baltimore, where officials estimate that 80 percent of the Maryland cases occur. In very young children, lead poisoning can permanently damage the nervous system, leading to mental retardation. At lower levels, lead poisoning can cause subtle learning, behavioral and psychological problems.

Experts believe that most children suffering from lead poisoning are never identified, largely because the symptoms are often vague.

Between 80 and 90 Baltimore children are hospitalized each year with serious lead paint poisoning, and physicians working with lead poisoning cases say they maintain about 450 active cases -- many of which are recurring cases several years old. More than 240 new cases were diagnosed last year. Most of those poisoned were poor children living in older houses with deteriorating paint.

A state task force on lead poisoning last year reported the cost of identifying and treating Maryland children poisoned with lead at more than $3.9 million a year.

There are also personal costs, besides worrying about medical treatment for a child whose symptoms of poisoning are vague or nonexistent. Some Baltimore property owners say they know of landlords who will not take tenants with very young children for fear of lead poisoning problems, while some will not rent if they believe a child has lead poisoning.

Lead is a cumulative poison. Small doses build up over time to create serious health problems, most of which, such as damage to the nervous system, are irreversible. In most cases, doctors concentrate on keeping the child away from the source of lead. In severe cases, children are hospitalized and given drugs that increase the excretion of metals from the body. But the treatment does not repair damage that has occurred.

People have suffered from lead poisoning since man started working with the metal. Some historians argue that the use of lead water pipes contributed to the decline and fall of the Roman Empire. Lead in gasoline, metal cans and paint has been dramatically reduced in recent years. But while cans and gasoline are used up, lead paint sticks on the walls.

In the 1950s, most paint companies reduced or eliminated the lead in house paint, where it was used to help color the paint and make it more opaque. It was outlawed in 1977.

Baltimore, which landlord representatives and others estimate has at least 100,000 houses containing lead paint, has laws forbidding chipped, flaking or peeling paint in rental houses, and compelling landlords to remove lead paint if a child has been poisoned. In other areas of Maryland, state law allows tenants to demand the removal of paint shown to contain lead, if it is within reach of young children. Baltimore is exempted from that law.

Several health officials and state legislators argue that these measures have done little to prevent lead paint poisoning. In Baltimore, they argue, nothing can be done until after the child is poisoned. In other areas of the state, little testing is done to find out where lead paint exists, so tenants have no reason to demand its removal.

A bill introduced in the General Assembly this year would require that the state provide lead paint testing by 1988 at a minimal charge to poor families -- and at cost to those who can afford it. If lead is present, children would be tested for lead poisoning; if the child is poisoned, landlords would be required to remove the paint.

The bill passed the House of Delegates March 17 and is being considered by the Senate Finance Committee, where it has faced opposition from landlord organizations. A second bill, passed by the committee Friday, would set up a lead paint council to study, among other things, ways of removing lead paint.

The landlords argue that testing houses for lead paint is a waste of government money because it is safe to assume that any house built before the mid-1950s contains it. And if the testing led to widespread paint removal, it would cost landlords so much that houses would be boarded up and the supply of low-rent housing would shrink. The landlord organization said it would cost at least $3,000 to remove lead paint from each house, a figure that is not disputed by advocates of lead paint removal.

"There's no economically feasible way to do the widespread abatement program," Ira C. Cooke, a lobbyist who represents the Greater Baltimore Property Owners Association, told Senate Finance Committee members last week. Instead, Cooke suggested, the state should concentrate on finding better ways to remove the paint, and on educating tenants, homeowners and parents on the dangers.

"It's unrealistic," said Stanley Sugarman, who owns and manages about 500 low-rent houses in Baltimore. If the walls are in good condition and parents are educated on how to avoid the dangers, he said, "there's no real danger. Many people live in these houses for years, and they don't get affected."

E. Clinton Bamberger Jr., a University of Maryland law professor who has represented tenants in lead paint lawsuits, said the landlords exaggerate the hardships that paint removal would cause. "At the moment, it's cheaper for them to poison the children than to remove the lead paint," he said. "What we have to do is to make it more expensive for them to poison the children."

Dr. J. Julian Chisolm Jr., director of the Lead Poisoning Clinic at the Kennedy Institute in Baltimore, agreed there are problems finding a safe and economical way to remove lead paint.

For the present, Chisolm said, there is little he can do but encourage the parents of lead-poisoned children to make sure their children do not put things in their mouths. "I urge them to move," he said, "but the fact of the matter is that they can't afford to move."