Pigeon bloom, a waxy powder which coats the feathers of racing pigeons, has been investigated as a potential antigen in pigeon fanciers' lung. Precipitins to a soluble bloom-extract were demonstrated in fifty-nine of ninety-seven people studied at two Scottish-pigeon shows, whilst precipitins to pigeon serum were found in thirty-seven. A radioimmunoassay was developed to allow an estimation of the intensity of antibody response to this antigen in nineteen pigeon fanciers being investigated for pigeon fanciers' lung. The mean antibody level to bloom extract was significantly higher (P less than 0.05) in those with acute symptoms compared to those with less severe symptoms. Pigeon bloom is a highly antigenic material and provides an alternative to pigeon droppings as a source for the inhalant antigens regularly encountered by those keeping pigeons.