L-374 II was an uninhabited planet. It was the second planet in the L-374 planetary system. This was one of two planets spared in the planet killer attack of 2267; the other was L-374 I. The other two planets in the system, L-374 III and L-374 IV, were destroyed.
When the crew of the badly-damaged USS Constellation was unaccounted for after the planet killer's attack, Spock considered it improbable that the ship's crew had fled to L-374 II as this planet had an atmosphere poisonous to Humans. (TOS: "The Doomsday Machine")