Can transfusions of “young blood” make old people healthier? That’s the hope of a few startup companies and some wish-to-live-forever billionaires who think transfusions from youngsters might delay aging.

But scientists in California who invented a gadget to test the question say instead it’s the harmful toxins in old people’s blood that seem to be the problem.

A research team led by Irina Conboy at the University of California, Berkeley, developed a pump to continuously move blood between two mice. One was young, the equivalent of a human 20-year-old, while the other was the mouse equivalent of an 80-year-old.

Over 24 hours, the two animals’ blood was completely intermixed.

Five days later, old mice did see some benefits from having young blood in their veins, including better muscle repair. But Conboy, who reported her findings in Nature Communications, says the really striking finding was just how bad old blood was for the younger animals. The aged blood inhibited the formation of brain cells in young mice and caused the animals to fall behind their peers in a strength test where they are hung upside down on a wire mesh. “The young mice became almost as decrepit as the old ones,” she says.

The research suggests that one day, instead of getting transfusions from young people, aged people will instead go to a medical facility to get their blood cleared of proteins that may build up and promote aging. Conboy says she and other scientists are working to identify what those molecules are.

Given the swift and negative effects of old blood on younger mice—the results appeared immediately—this type of research could eventually raise questions about the age of blood-bank donors. A 2008 study in Blood found that the average age of blood donors in the U.S. was 35, but since repeat donors tend to be older, about 35 percent of blood came from people over 50, including many in their 60s.

Conboy says there isn’t yet cause to worry about giving a teen blood from a grandparent. One reason is that no one knows how long the negative effects of a single “old-blood” transfusion would last. What’s more, blood transfusion is often a life-saving intervention. That kind of benefit “should not be overshadowed by any doubt” about the age of the blood donor, she says.

The possibility that blood harbors the causes of old age—or an elixir of youth—is one of several new avenues of anti-aging research that is attracting attention. Others include pills that change people’s metabolism or drugs that disintegrate cells that have stopped functioning. Conboy’s study was funded in part with a $120,000 scientific grant from Calico, the secretive anti-aging startup created by Google in 2013.

The new work builds on previous attempts to rejuvenate old animals by surgically joining them with young ones so that their blood would mix. That procedure, called parabiosis, proved fairly successful at rejuvenating older mice, and sparked the idea that transfusions of blood or serum from young people might delay aging or Alzheimer’s, a tactic now being tested in at least two clinical trials in the U.S.

However, Conboy strongly criticized the “young blood” studies as premature given the early stage of scientific research. “I don’t think that there is any scientific justification that it would work,” says Conboy. “Taking a young person’s blood and infusing it into an old person is not medicine.”

In part, that’s because with parabiosis, animals share much more than blood. They share livers, immune systems, even the same blood pressure and temperature. Ben Alman, chairman of the orthopedics department at Duke University, says Conboy’s device is an advance because it could help scientists zero in on the effects of blood alone.

Alman notes that Conboy’s team did see some benefits to the older mice, although not as great as in the parabiosis experiments. “What we know is there are all sorts of factors in blood that will make animals behave the age of the blood rather than the age of the animals,” says Alman.