Paul Ehrlich had just injected aniline dye--used to color blue jeans--into a rat's bloodstream. For years the immunologist had been working on ways to stain cells so they would be more visible under a microscope, and aniline looked promising. Soon all the animal's muscles, blood vessels and organs were deep indigo. But for some confounding reason the central nervous system--the brain and spinal cord--remained untouched.