Destroy J-Lab! is a mission in Grand Theft Auto 2 given to Claude Speed by Elmo of the Loonies. The mission is available from the right of the two yellow Loony phones in Fruitbat of the Downtown District.
The Yakuza have created a drug to cure the insane, and are using it on the Loonies. They would like to stay in insanity, and so Elmo hires Claude to blow up the Yakuza's J-Lab, where the stuff is being developed. Claude does this and ends up destroying the place and project permanently.
The reward for completing the mission is $40,000.