A Will Cornell Medical College research team says they extracted early-stage sperm cells from mice and then turned them into cells capable of becoming different tissues.
Writing in Nature, the team hoped their work could lead to treatments for illnesses such as Alzheimer's and diabetes.
Stem cells are the body's "master cells" that, in theory, can become any type of cell in the body.
An obvious source of these is from the human embryo, as unlike adult cells, these have the potential to grow into any tissue type.
However, ethical concerns over the use of embryos in medicine mean that scientists are hunting for a source of easily-harvested adult cells which could be coaxed into any variety of cell.
Stem cells have already been extracted from mouse testicles - however, the New York team is claiming a more reliable way to isolate and develop them, increasing the potential for larger numbers to be produced successfully.
The testicular cells do not need to be genetically "tweaked" to behave more like embryonic stem cells, unlike other "adult stem cells" found elsewhere in the body, say the scientists.
Dr Shahin Rafii, who led the research, said: "It appears that these unique specialized spermatogonial cells could be an easily obtained and manipulated source of stem cells with exactly the same capability to form new tissues that we see in embryonic stem cells.
"For male patients, it could someday mean a readily available source of stem cells that gets around ethical issues linked to embryonic stem cells.
"It also avoids issues linked to tissue transplant rejection, since these 'autologous stem cells' are derived from the patient's own body."
He listed several illnesses which he hoped could be tackled using stem cell technology, including Parkinson's Disease, Alzheimer's, stroke, diabetes and even certain cancers.
It is hoped that one day, implanting large quantities of stem cells into tissue damaged by disease could prompt the body to replace it.
Professor Colin McGuckin, a researcher in stem cell biology at the University of Newcastle, said that several research teams around the world were looking into the potential of the testicle as a stem cell source.
He agreed that there was a lot of interest in such possibilities, especially among veterinary circles as a source of stem cells for animal use.
But also cautioned that not too many men might be willing to undergo the procedure to extract the cells, for such extraction would be a very painful procedure.