A new strain (D61) of EDS-76 virus associated with an egg-drop syndrome in chickens was compared by neutralisation and haemagglutination-inhibition (HI) tests with two recognised strains (127, BC14). All three strains were serologically indistinguishable. Each strain lacked any demonstrable ability to lyse the red cells of chickens or ducks. To investigate its pathogenicity and immunogenicity, chickens of different ages were inoculated with the D61 strain per os. All birds produced eggs with aberrant shells in varying numbers, but internal quality was not affected. There were no other constitutional effects and no obvious clinical signs throughout. Fertility and hatchability remained unaffected. At 19 weeks, onset of laying was delayed by 9 days, and total daily egg production was eventually 15 to 20% lower than that of uninfected controls of the same age. The total daily egg production of older chickens was unaltered, except for approximately 2 weeks after infection at 26 weeks of age and for 5 weeks at 47 weeks of age. The highest number of eggs with abnormal shells was laid by chickens infected at 33 weeks (up to 38%) or 47 weeks of age (up to 40%). Infective virus was successfully recovered from the livers of chicks immediately after hatching from eggs laid by hens 7 to 11 days following infection. Neutralising, HI and precipitating antibodies developed within 6 days of infection, and were associated with the IgG class of immunoglobulins. Chicks hatched from eggs of infected hens possessed maternal antibodies with a half-life of 3 days; these antibodies conferred a passive immunity to challenge for approximately the first 4 weeks of life, based on the evidence of HI responses after challenge.