A mercury which has been used in vaccines stays in the blood for less time than mercury found in fish from pollution, a study says.
Thimerosal is being phased out of vaccines used in the UK
But the researchers said it did not mean thimerosal, a mercury-based preservative used in vaccines which has been linked to autism, was harmless.
And the University of Washington team added more research was needed to establish the toxicity of thimerosal.
Thimerosal is being phased out of use in vaccines in the UK.
The new five-in-one vaccine to immunise babies against polio, diphtheria, tetanus, whooping cough and hib introduced last autumn replaced a jab which contained mercury.
Only hepatitis B and influenza in the UK programme now contain thimerosal.
The US team tested 41 newborn monkeys, feeding them either methyl mercury, which is found in fish from pollution, and ethyl mercury, contained in thimerosal.
They found the ethyl mercury was cleared from the blood much more quickly and the levels in the brain were much less.
However, the study, published in the journal Environmental Health Perspectives, also found that a much higher percentage of mercury in the brain was in the form of inorganic mercury for thimerosal-exposed monkeys.
Report co-author Thomas Burbacher said: "There is both good news and bad news here.
"The current debate linking the use of thimerosal in vaccines to autism and other development disorders has led many families to question whether the potential risks associated with early childhood immunisations may outweigh the benefits.
"What we need to do now is assess the toxicity of the mercury and, in particular, inorganic mercury."
Isabella Thomas, of campaign group Jabs, said it was important to remember the dangers associated with mercury in vaccines.
"I think we are going to hear much more about the dangers of mercury.
"The problem is that we hear so much conflicting evidence from research.
"I think it is right mercury is being taken out."
And a spokeswoman for the Medicines and Healthcare products Regulatory Agency, the UK drugs regulator, said mercury was being phased out of the UK vaccine programme after advice from the World Health Organisation.
She said there was no proof of a safety concern, and the UK was just acting as a precaution, adding mercury was only taken out if it did not compromise the jab.
"There is strong evidence to show that thimerosal in vaccines does not cause neurodevelopmental problems."