A rep for the animal rights org. tells TMZ ... "Neuralink and the University of California-Davis are responsible for the deaths of at least 15 monkeys because of Elon Musk’s quest to stick a chip in a human’s brain."

PETA says this kind of experimentation dates back at least 2 decades at other companies -- but notes, in just a few short years, Neuralink is piling up animal corpses all on its own.

The org adds, "Experimenting on and killing animals is never good science, and since monkeys in Musk’s experiments have also died for reasons that have nothing to do with these studies, the laboratory should be closed."

PETA's solution, we're told, includes urging the USDA's inspector general to get to the bottom of why animal protection laws were allegedly violated -- as cited in a Reuters report, which detailed in-house workers' concerns with the growing number of animal tests and deaths. Reuters says at least 1,500 animals have been killed at Neuralink since 2018, in the advancement of research, and to meet Musk's alleged timetable for the chip.

Now, PETA's looking to put the pressure on Elon by using his new $44 billion toy against him -- it's calling on Twitter users to flood the app with protest tweets tagging @elonmusk and his company @neuralink.

Neuralink has made promises of being able to help combat paralysis by allowing patients to simply think out a task that machinery would then complete.

Earlier this year, there was some indication Elon would start human trials, but it doesn't seem that's happened yet.