Six commercial infectious bronchitis virus (IBV) vaccines and five IBV field strains were titrated in tracheal organ cultures (TOC) and oviduct organ cultures (OOC). Endpoints were determined in three different ways: ciliostasis (CD50), immunofluorescence staining (IFID50) and organ culture infectivity (OCID50). For the two most attenuated vaccine viruses, infectivity assessed by IFID50 and OCID50 was significantly higher than that assessed by CD50. No significant differences were found between IFID50 and CD50 with the five virulent viruses in either OOC or TOC. This suggests that the most attenuated viruses multiply in these tissues to a greater extent than assessed by their pathogenic potential i.e. ciliostasis. Comparison of IFID50 and CD50 may be a useful method for screening candidate live respiratory viral vaccines for attenuation. OOC prepared from oestrogen-treated embryos were not efficacious.