1. Allan, W. H., andC. N. Herbert: The precision of virus end-point determinations. Arch. ges. Virusforsch.25, 330?336 (1968).
2. Almeida, J. D., D. M. Berry, C. H. Cunningham, D. Hamre, M. S. Hofstad, L. Malucci, K. McIntosh, andD. A. J. Tyrell: Coronaviruses. Nature (Lond.)220, 650 (1968).
3. Asplin, F. D.: Identification of infectious bronchitis of chickens in England. Vet. Rec.60, 485?486 (1948).
4. Beaudette, F. R., andC. B. Hudson: Cultivation of the virus of infectious bronchitis. J. Amer. vet. med. Ass.90, 51?60 (1937).
5. Beery, D. M., andK. J. Stokes: Antigenic variations in isolates of infectious bronchitis virus. Vet. Rec.82, 157?160 (1968).