The selective damage of cell structures by partial irradiation is a technique employed by biologists since the early 1900's.(See reviews: Refs. 1, 2, 3, 4) The approach is simple: expose a specific cellular region or structure to a non-lethal dose of radiation, and then examine the cell with respect to altered structure and function. In this way, much can be learned about various subcellular structures (organelles) and general cell function. In theory, this approach to the study of organelle function could be most productive. Indeed, several hundred studies have been published. However, of these only a few have resulted in substan-tial progress in understanding the cell. The types of radiation have included particulate (protons, electrons, alpha particles, etc.) and electromagnetic (ultra-violet, X-ray, gamma ray). It is probable that the general damaging properties of these types of radiation have resulted in much secondary cell damage, thus making strict interpretation of the results difficult. It was not until the early 1960's that successful partial cell irradiation was performed with visible wavelengths. The intensity attainable at a single or few wavelengths with laser light has permitted the selective damage of numerous cell organelles. Surprisingly, secondary effects to non-target structures appear to be minimal , if not non-existent. Though a coherent explanation of this capability cannot be given at this time, the fact remains that numerous cell structures can be affected selectively without apparent damage to nearby organelles. These results have been confirmed by assaying subsequent cell function as well as cell ultrastructure. In fact, it has been possible to demonstrate that the unirradiated regions of an otherwise irradiated organelle are structurally (and apparently functionally) unaffected.