A method destroys or impairs target cells that have selectively accumulated a photosensitizing agent. The target cells are in the bloodstream of an intact animal, which bloodstream and animal further contain non-target cells. Radiation is applied transcutaneously to at least a portion of the intact animal at an intensity effective to impair or destroy selectively the target cells and to leave non-target cells relatively unimpaired. Target cells include leukemia cells, virus-containing cells, parasite-containing cells, and microorganisms such as bacteria, parasites and free viruses.