The mammalian olfactory epithelium consists of supporting cells, olfactory receptor cells, basal cells and a fourth cell type, which has recently been discovered. In this study we examine this fourth cell type using immunohistochemical techniques. Anti-Purkinje-specifïc spot-35 protein (S-35), anti-S-100 protein (S-100), anti-neuron-specific enolase (NSE), antichromogranin A and antisynaptophysin antisera were used for the immunostaining. The fourth cell type immunoreacted only to anti-S-35 antiserum and did not react to other antisera in guinea pigs. On the other hand, in the human S-100 immunoreactivity was seen in the fourth cell type as well as weak reactivity to S-35. NSE immunoreactivity was found only in the olfactory receptor cells of guinea pigs and the human. From these results it is assumed that this fourth cell type is a second chemoreceptor different from the olfactory receptor cells because S-35 and S-100 are Ca++-binding proteins