Natural resistance is now coming to be recognized as a potentially important phenomenon in host defense against infection and ma- lignancy. Genetically controlled resistance mechanisms are usUally effective early in infection and before conventional immune responses are generated. Comparisons of experimental systems where natural resistance plays a prominent role demon- strate the complexities of the host defense mechanisms involved, as evidenced in the present volume. Nevertheless, some com- mon components of genetic resistance are discernible and largely comprise natural killer cells,...