Several investigators have reported that the cellular immune response to an antigenic stimulus has greater specificity than the antibody response (1,5,10). Following the description of the macrophage migration-inhibition (MMI) test (6), an in vitro method for detecting delayed hypersensitivity, reports have appeared in the literature describing MMI responses to various antigens; however, studies with viruses have been limited and the results obtained variable. For example, Feinstone et al. (4) reported that the MMI test was both a sensitive and specific test for the assay of influenza and mumps viruses; however, Tremonti and Jackson (11) reported difficulty in detecting MMI responses to mumps, parainfluenza, and respiratory syncytial viruses using peritoneal exudate cells from sensitized guinea pigs. This report describes MMI test results obtained with two serotypes of infectious bronchitis virus (IBV).