Two Americans have won the 2004 Nobel Prize in physiology for their work in studying odorant receptors and the organisation of the olfactory system in human beings.

Richard Axel, 58, of the Howard Hughes Medical Institute at Columbia University in New York, shared the prize with Linda B Buck, 57, of the Fred Hutchinson Cancer Research Centre in Seattle, Washington.

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The prize includes a cheque for $1.3 million.

Their work "discovered a large gene family, comprised of some 1,000 different gene (3 per cent of our genes) that give rise to an equivalent number of olfactory receptor types", the foundation said.

Last year's prize winners were Briton Sir Peter Mansfield and American Paul C Lauterbur for discoveries that led to the development of MRI, which is used by doctors to get a detailed look into their patients' bodies.

The award for medicine opens a week of Nobel Prizes that culminates on October 11 with the economics prize.