A study published Wednesday found that rapamycin, a drug used in organ transplants, increased the life span of mice by 9% to 14%, the first definitive case in which a chemical has been shown to extend the life span of normal mammals.

Anti-aging researchers also expect a second study, to be released this week, will show that sharply cutting the calorie intake of monkeys extends their lives substantially. The experiment is said to be the first technique shown to retard aging in primates.

The prospect of a reliable human longevity pill is still distant. A commentary released with the rapamycin study strongly cautioned against taking the drug to prolong life because of potentially deadly side effects. Rapamycin suppresses the immune system and carries strong warnings about the resulting risk of infections and death.

But the mouse and monkey findings appear to mark the most substantial scientific progress yet in the search for ways to extend human life spans -- once viewed as a fringe area of study.

'It's time to break out of our denial about aging,' said Aubrey de Grey, a British gerontologist who has drawn controversy for his suggestions on how to forestall death. 'Aging is, unequivocally, the major cause of death in the industrialized world and a perfectly legitimate target of medical intervention.'

The studies boost the notion that restricting metabolic activity -- whether through a drug or calorie restriction, which involves sharply reducing food intake -- lengthens life span.

'It's all consistent with what human practitioners of calorie restriction have always believed,' said Brian Delaney, president of the North Carolina-based Calorie Restriction Society, which claims 3,000 members. 'Any degree of restriction beyond what you're currently eating will confer health benefits and will slow the aging process.' Many of the society's members restrict their eating to a level not much above starvation levels in the hope of living longer.

The monkey study, which will be published in Science magazine, will report on about 40 rhesus monkeys at a Univerity of Wisconsin lab who have been calorie-restricted for 20 years. Several longevity researchers who have seen pre-released copies of the study described its findings.

The mouse study, published in the journal Nature, is part of a systematic search by the federal government's National Institute on Aging, and was conducted by biologists and university researchers in Maine, Michigan and Texas. The study group has so far tried seven different chemicals, including aspirin and the cardiovascular drug enalapril, generally with middling results.

But rapamycin, a fungus-fighting bacterial secretion originally discovered in the soil of Easter Island, showed a marked effect. The compound is branded by drug maker Wyeth as Rapamune and is used to suppress the immune system and ward off rejection after organ transplants and in heart stents.

A Wyeth spokesman called it an 'interesting preclinical study' and said Wyeth just became aware of the finding Wednesday.