The nature of cell-mediated immunity in the American cockroach (Periplaneta americana) was studied by observing the events involved in graft reactivity. Adult roaches received integumentary xenografts from Blaberus giganteus, as well as control autografts. Grafts were scored histologically by counting the number of nuclei in a 100-mu-wide area under the center of the graft. The data showed that the reaction to the xenografts was characterized by quick recognition of foreignness by day 3 posttransplantation, followed by a slow decline in the cellular response, which lasted for more than 70 days. Control autografts did not appear to stimulate a reaction. Histological examination of xenografts revealed a disruption of the normal cytoarchitecture by a heavy cellular infiltrate. Our findings indicate that the outcome of the response to foreign integument in the roach is more characteristic of an encapsulation reaction than a classic rejection reaction.