Medicinal microrobots could help physicians better treat and prevent diseases. But most of these devices are made with synthetic materials that trigger immune responses in vivo. Now, for the first time, researchers have used lasers to precisely control neutrophils — a type of white blood cell — as a natural, biocompatible microrobot in living fish. The ‘neutrobots’ performed multiple tasks, showing they could someday deliver drugs to precise locations in the body.