Mice kept in the deep freeze for 15 years have fathered healthy offspring, say scientists in Japan and Hawaii.
Some of the mice were fathered by adults that died 15 years ago
One in five female mice undergoing IVF with sperm extracted from the dead mice had healthy, fertile pups.
It offers hope to those trying to bring extinct animals back from the dead, they report in the Proceedings of the National Academy of Sciences.
One idea would be to inject frozen mammoth sperm recovered from the ice into the eggs of female elephants.
The researchers wrote: "If spermatozoa of extinct mammalian species (eg woolly mammoths) can be retrieved from animal bodies that were kept frozen for millions of years in permanent frost, live animals might be restored by injecting them into oocytes from females of closely related species."
The team, led by Atsuo Ogura of the Institute of Physical and Chemical Research Bioresource Center in Tsukuba, Japan, harvested sperm from intact testes and, in some cases, the entire mouse, of specimens stored at -20C for up to 15 years.
They did not use hi-tech cryo-protection techniques, simply storing whole testes or bodies in a freezer.
The sperm appeared lifeless when thawed out but researchers were surprised to find that they produced viable offspring.
However, the chances of being able to recreate a woolly mammoth - or at least, an elephant with a woolly mammoth father, are thought to be slim.
Any mammoth recovered from the permafrost would have spent more than 10,000 years in a frozen state.
It would also have been frozen relatively slowly, making it liable to damage from bacteria and other micro-organisms.