DN6 was a pesticide developed in the early 1960s by Smithers to end famine.
In June 1969, Arnold Farrow discovered it worked so effectively that, if used widely, it would have caused serious harm, destroying insects vital to Earth's ecology as well as those harmful to it. Forester, who had invested heavily in DN6, murdered Farrow to cover up his findings. His crimes were exposed thanks to the secret help of the First Doctor and his companions, but not before Barbara Wright was briefly poisoned by DN6. (TV: Planet of Giants)
The DN6 was confiscated by Alistair Gordon Lethbridge-Stewart's fledgling Fifth Operational Corps, when he and Anne Travers were called in to investigate. (PROSE: House of Giants) It was later used to destroy the Grandfathers' pods on Strommach. (PROSE: The Grandfather Infestation)