Proteins associated with the specific functions of differentiated cells have been found in numerous systems. By means of two-dimensional polyacrylamide gel electrophoresis, we have identified two polypeptides, apparently specific to the organ of Corti and tentatively named OCP-I and OCP-II. These polypeptides comprise a substantial proportion of the total protein of the organ of Corti of the guinea pig and rat, the only species studied to date. They are not present in other inner ear structures (macula sacculi, tectorial membrane, stria vascularis, etc.), nor in other tissues (brain, liver, retina, etc.). The molecular weights of OCP-I and OCP-II are 37 000 and 22 500 daltons, respectively. The corresponding pI values are 4.9 and 5.1. At this preliminary stage we can only speculate about the functions of OCP-I and OCP-II; it is conceivable that they are involved in auditory transduction and/or transmission. [Supported by NIH.]