Comparative immunologists have been engaged in a search for the origins of cell-mediated immunity for a long time. One group of invertebrates that have been receiving increasing attention has been the insects. These animals not only serve as good experimental models, but are of great interest because of their evolutionary success. Several studies have shown that insects generally respond to particular foreign material by mounting an encapsulation reaction. On a more specific level, insects have also been found to be capable of quickly rejecting integumentary xenografts. A basis enigma however, has been the reported inability of insects to respond to integumentary allografts. Recent work from our lab indicates that if one concentrates on the fate of the soft tissue component of the integument, one can readily demonstrate allograft reactivity in these animals. We have now confirmed this by yet another system which monitors the loss of 3H-thymidine from labeled allo- and autografts. Thus, future studies promise to unravel some long sought after questions concerning the evolution of cellular defense mechanisms.