AbstractTwo monoclonal antibodies and serial section analysis have been used to compare the sites and times at which hair and supporting‐cells differentiate in various sensory regions of the chick inner ear during its development. A monoclonal antibody recognising the 275 kD hair‐cell antigen, a protein that is specifically associated with the apical surface of hair cells, was used to identify immature hair cells. Another monoclonal antibody, gm‐2, which stains the gelatinous membranes of the cochlear duct, sacculus, and utriculus and the epithelial supporting cells of all vestibular structures in the inner ears of early posthatch chicks, was used to detect the onset of supporting‐cell differentiation. Although the antigens recognised by the two antibodies are first detected almost simultaneously during the development of each sensory region, their appearance is not always exactly temporally coincident, and their order of appearance, when not coincident, varies between the epithelia but is always the same within any one organ. Also, when the two antigens are first present in any one region, there is not always a very precise overlap in their spatial distribution. These results cannot be explained entirely by a previously proposed model for hair and supporting‐cell development in which hair cells differentiate first and prevent undetermined, surrounding cells from becoming hair cells via lateral inhibition. Modified forms of the original model that can accommodate some of these observations are considered and discussed. © 1995 Willy