Nylon monofilaments implanted in the hemocoel of the American cockroach (Periplaneta americana) were reacted against and completely encapsulated by hemocytes. No encapsulation of implanted nerve cords exchanged between individual cockroaches of this species occurred. Interspecific implants between two species of cockroach (P. americana and Nauphoeta cineria) were recognised as foreign and encapsulated, as were implants from an unrelated insect (Calliphora), and from mice. Homologous nerve cords were treated with various enzymes in attempts to render them susceptible to encapsulation. Positive results were obtained with collagenase and lecithinase C. The effects obtained were consistent with the idea that an intact neural lamella is required to avoid the hemocyte reaction.