Injection of guinea-pig cytomegalovirus into seronegative guinea pig cochleas results in viral labyrinthitis. The location of infected cells 1-8 days following inoculation was examined using an immunohistochemical assay for viral antigens. Mesothelial cells below the basilar membrane and those lining the scala tympani, especially below the osseous spiral lamina, were the first cells to express antigen. This occurred 2 days following inoculation. Infection then spread rapidly toward the brain and through the perilymphatic duct to involve inflammatory cells, mesothelial cells of Reissner's membrane, spiral ganglion cells, and the vestibular ganglion cells. In any one cochlea, more mesothelial cells and inflammatory cells expressed viral antigen than any other cell type. It seems then, that there is a differential susceptibility to viral infection and antigen expression in the various cell types of the cochlea. Typical cytomegalic inclusions were common in mesothelial cells. However, many cells labelled for viral antigens did not contain inclusions. The identification of viral antigens then, is a more sensitive technique for demonstrating infection than is the location of cytomegalic inclusion cells.