THE RESPONSE of susceptible chickens to inoculation with some commercially available infectious bronchitis (IB) vaccines formed the subject of investigation of the first paper of this series (Raggi and Bankowski, 1956). The main objective of the present study was to investigate whether or not the pathogenicity of the above IB vaccine strains for chicken embryos was sufficiently characteristic for use as a criterion to differentiate a field strain from a vaccinal strain of IB.An attempt was also made to stabilize the time and the pattern of death of normal chicken embryos inoculated with one of the strains (No. 6) of infectious bronchitis virus (IBV).MATERIALS AND PROCEDUREIB vaccinal strains obtained directly from the manufacturers were stored at 2°C. for a few days before being tested. The vaccines were reconstituted by aspirating the diluent with a sterile syringe and needle inserted through the rubber stopper previously swabbed with 95 …