Avian infectious bronchitis virus strain Kagoshima-34 isolated from the kidneys of a chicken that died of nephrosis/nephritis lost its nephropathogenicity during intratracheal passage in SPF chickens. The resultant virus acquired stronger respirotropism but reduced tropism for kidneys. On the other hand strain Tottori-2 isolated from the trachea of a chicken suffering from severe respiratory disease did not lose its respirotropism after serial intravenous passage in SPF chickens. The serological properties of the passaged virus were investigated by virus neutralisation test. The antibody titres of both strains of virus fluctuated with progressive passage. The serological properties of the virus isolated from respiratory organs were not necessarily the same as those of the isolates made from the kidneys.