We have utilised a novel in vitro encapsulation system using haemolymph of the host, Heliothis virescens and eggs of the parasitoid, Cardiochiles nigriceps (as targets for encapsulation) to examine the properties of the egg surface with respect to recognition by the host haemocytes. Many biotic and abiotic targets were readily encapsulated in vitro, with the exception of negatively charged cation exchange beads and allogenic tissue. However, mature eggs, which had a 0.5-1.0 mu-m thick fibrous layer on their outer surface, were not encapsulated in vitro regardless of whether they were developing, dormant or had been killed by ultra-violet irradiation. Conversely, immature eggs, which had a sheath of follicle cells over over the fibrous layer, were readily encapsulated; similarly, experimental removal of the fibrous layer with driselase or H. zea haemolymph also caused the eggs to elicit an in vitro encapsulation response. Identical encapsulation responses occur if eggs are injected manually into H. virescens larvae. The data suggest that eggs of the parasitoid, C. nigriceps can evade encapsulation by the host haemocytes independently of other "immunosuppressants" such as calyx fluid/virus, venom or teratocytes. It is proposed that the fibrous layer may, among other possible functions, delay encapsulation until a more permanent means of suppressing encapsulation of the egg is established in the host. none