Scientists say they have isolated the natural chemicals which make some people smell unpleasant to mosquitoes.
Antennae gave clues to the insect's response (Royal Society)
The team is not naming the chemicals involved but hopes a new generation of natural insect repellents may follow.
Blood-sucking insects play a role in spreading many different diseases, and any new way to minimise their threat could potentially save many lives.
The work was carried out by Rothamsted Research in Hertfordshire and the University of Aberdeen.
Mosquitoes only target people who smell nice - leaving those with unattractive odours alone.
But humans produce hundreds of different chemicals, so it has proved difficult to tease out which turn the insects' stomachs.
The researchers used a two-stage process to solve the problem.
Gas chromatography enabled them to break down human odours into their individual chemical components.
Then a second technique, called electroantennography, allowed them to simultaneously record the responses in the mosquito's antennae.
The scientists are refusing at this stage to give more details of the key chemicals.
Lead researcher Professor John Pickett said: "We are currently testing the natural compounds by comparing them with World Health Organization approved insect repellents on individuals known to be attractive to mosquitoes.
"We hope to publish the results of these trials soon."
Professor Pickett said many current mosquito repellents were based on plant chemicals, which could cause dermatological problems in users, and often had a strong smell.
He said the main alternative, DEET, could accumulate in the body's fat tissue, and was also irritating to the eyes.
"The chemicals we have identified are active at phenomenonally low levels which are subliminal to our perception," he said.
Dr Ron Behrens, of the London School of Hygiene and Tropical Medicine, said the study sounded technically impressive.
However, he said there were already very effective insect repellents available - the problem was getting people to use them properly.
"What would really count would be if they could come up with a formulation that you use once, and it lasts for a long time," he said.
The research was presented at the Royal Society Summer Science Exhibition which opened today in London.