The differentiation of the cuticular plate and stereocilia in cochleovestibular hair cells from the mouse was traced with monoclonal antibodies raised by in vitro immunization. The cuticular plate is detected first from embryonic days 14-15 (E14-E15), before cell differentiation is apparent, either with scanning electron microscopy or with actin filament labeling. A flat disc of material forms beneath the apical membrane and subsequently expands, forming a fully shaped cuticular plate at postnatal stages 3-5 (P3-P5). A second antibody labels stereocilia from stage E16 to E18. In the cochlea, the label initially appears as a punctate disc on the cell apex and then follows the development of the stereocilia until the adult shape of the bundle forms at P4-P6. Additional antibodies label stereocilia from P4 to P6 and are apparently specific for the inner ear. They do not label the cuticular plate at any stage and do not cross react with tissues of muscle, kidney, eye, tongue, gut, skin, or brain. At stage P12-P14, coinciding with the functional maturity of the ear, they label the apical regions of Deiter's cells. The temporally overlapping sequence of antibody labeling sheds new light on the development of the hair cell apex and allows us to monitor the differentiation of hair cells from their last mitotic division to the initiation of organ function, a period of over 2 weeks.