The Massachusetts and the Connecticut types of infectious bronchitis virus (IBV) were identified by interference in embryonating chicken eggs (ECE) with the production of hemagglutinin by the B-1 isolant of Newcastle disease virus (NDV). This interference test appears to be specific because the above interference was eliminated by adding type-specific anti-IBV serum to the IBV-NDV system; however, interference was not detectable when fowlpox virus (FPV) and infectious laryngotracheitis virus (LTV) were substituted for IBV. Specificity of the interference test was dependent upon a system involving IBV, NDV, FPV, and LTV. The test can be done in 3 days and requires minimum laboratory facilities. Most of the experiments were done with the Massachusetts type of IBV. Only a few were with the Connecticut type. The interfering action of the above two types of IBV over the B-1 isolant of NDV waxed between the 24th and 54th hr after inoculation of NDV; it was waning at the 54th-60th hr postinoculation and was undetectable by the 66th-72nd hr.