When Eldrad returned to Kastria after a one hundred and fifty million year absence, and in a female form, he triggered a booby trap that pervaded his form with acid designed to neutralise the molecular bond. With no antidote in existence, Eldrad asked the Fourth Doctor to take him to the regenerator chamber.
The device had been programmed by the Kastrians to obliterate Eldrad on his return, but it instead regenerated him, restoring his true male form. Eldrad explained to the Doctor and Sarah Jane Smith that he had programmed the regenerator to recognise his cellular pattern and thus it was incapable of destroying him. (TV: The Hand of Fear)