A condition showing anaemia with asplasia of the bone marrow and atrophy of the lymphoid organs occurred in young layer chickens on a poultry farm. An agent (tentatively designated TK-5803 strain) was isolated in specific-pathogen-free (SPF) chicks from livers of the field cases. The agent was severely pathogenic when inoculated into 1-day-old SPF chicks, but the chicks developed age resistance to the agent. The agent produced no cytopathic effect nor alterations in chick embryo fibroblasts and chick kidney cells. Antisera to the agent had neutralising antibodies but no activity against other infective agents. The agent was resistant to ether, chloroform and pH 3.0, passed through a 25 nm membrane filter, and was inactivated partially after heating for 30 min at 80 degrees C and completely after 15 min at 100 degrees C. These characteristics of the agent were similar to those of the chicken anaemia agent reported by other workers.