Egg drop syndrome 76 (EDS 76) is a disease of chickens caused by an haemagglutinating group III aviadenovirus (Smyth and McFerran, 1989). Infection with the virus causes a failure to reach peak production in flocks affected prior to 20 weeks of age, or a decline in production of from 10% to 30%, accompanied by the presence of soft- shelled eggs in flocks infected in early lay (McFerranet al., 1978). The disease was reported to have first occurred in the Netherlands in 1974 (van Ecket al., 1976), and the causative agent identified in Northern Ireland in 1976 (McFerranet al., 1978). Antibodies to EDS 76 virus have been detected in duck sera collected prior to 1976 (McFerranet al., 1977), but not chicken sera, and the preference of the virus for duck cells has lead to the suggestion that EDS 76 is a duck adenovirus introduced to chickens, probably via a contaminated vaccine (Smyth and McFerran, 1989). The use of contaminated vaccines in primary breeding stock lead to the dissemination of the virus through chicken populations by vertical transmission, and the first report of EDS 76 in New Zealand commercial egg layers was traced to infection in a parent breeder flock (Howell, 1988). Subsequent reports of EDS 76 in New Zealand were linked with exposure of free-range egg layer flocks to water contaminated by duck faeces (Christensen and Stanislawek, 1994), and in 1995-96 EDS 76 was shown by EDS HI tests to have became established on a number of free-range egg laying fams, despite efforts to limit contact with water contaminated with duck faeces. This is thought to be the method of introduction of infection to the trial farm. These seropositive flocks, including the unvaccinated flocks shown in Figure 1, suffered declines in production typical of EDS 76.