Scientists say they found "no difference between sperm frozen for 50 years and sperm frozen for a year".
Endangered species may have found a new lifeline after sheep were impregnated using 50-year-old frozen sperm.
Samples taken from four rams including one called Sir Freddie in 1968 are described as the "world's oldest known viable semen".
Scientists from the University of Sydney inseminated 56 Merino ewes with the half-century-old semen, and 34 became pregnant.
The 61% success rate was higher than that of sperm frozen for only a year – which impregnated 59% of inseminated ewes.
Associate Professor Simon de Graaf said the trial demonstrated the "clear viability of long-term frozen storage of semen", raising new hope for the propagation of endangered species.
The semen was stored as small pellets in large vats of liquid nitrogen at -196 degrees, the university said.
After it had been defrosted, post-doctoral research fellow Dr Jessica Rickard tested its motility, velocity, viability and DNA integrity.