Methods for distinguishing between two interspersed biological tissues, for procedures such as surgical resection, include exposing the tissues to at least two components, a first of which components produces or is capable of producing a detectable signal, and the other of which components either blocks the produced signal of the first component or activates the first component to produce the detectable signal. One of the components is selectively taken up by one of the tissues at a concentration which is greater than the concentration by which it is taken up by the other tissue to provide a distinguishable difference in the detectable signal originating from the two tissues.
