The TK-5803 strain of an agent which induces chicken anaemia was propagated and titrated in cultures of the MDCC-MSB1 cell line. When the isolates (tentatively designated MSBI-TK-5803 strain) from the original material were inoculated into 1-day-old susceptible chicks, they showed a severe pathogenicity, inducing anaemia with aplasia of the bone marrow and atrophy of the lymphoid organs. On density gradient separation, the peak of their infectivity titre appeared at a density of 1.35 to 1.36 g/cm(3) and numerous virus particles were demonstrated in the same fraction. The agent was resistant to heating for 30 min at 60 degrees C and 50% chloroform for 15 min at 4 degrees C, and passed through a 25 nm membrane filter, but it was completely inactivated after heating for 30 min at 70 degrees C or more. The purified virus particles were spherical or hexagonal in shape and 19.1 +/- 0.2 nm in diameter.