Village of the Giants (1965)

Mike thinks he's hit the jackpot when his girlfriend Nancy's kid brother, who they call Genius, creates a growth compound that results in animals growing to huge proportions. A group of delinquents led by Fred has come to town and when they get their hands the goo, as Genius calls it, their giant size make them unstoppable. While Genius tries to recreate the formula, Mike and other teenagers in town, serve the giants. Genius simply can't reproduce the formula but fortunately comes up with the next best thing.