THIS investigation was designed to clarify some confusing statements concerning rhinoceros horn which appear in the literature : for example, that it appears to be composed of matted hair1,2; that it is formed wholly of keratin produced by stratum corneum3; that it is composed of coarse keratin fibres cemented together4. The present investigation has confirmed that rhinoceros horn has a tubular (filamentous) structure comparable with that described in other horn by Trautmann and Fiebiger5 quoting Nickel6. This structure of rhinoceros horn has already been briefly recognized by Boas7, whose term for the tubules was canals, by Makinson8, who described the structure as cylindrical, and by Le Gros Clark9. But, as horn tubules (filaments) in general form over dermal papillae, the implication of Le Gros Clark that rhinoceros horn is unique in this respect is not correct. Dove10 said that rhinoceros horn was similar to that in cattle. During the present work it has not been possible to demonstrate horn, tubules in either sheep or cattle horns, in which, according to Trautmann and Fiebiger, the tubular structure is not marked; in addition to those in rhinoceros horn, tubules have been observed in the hoof. In the present work the term filament is preferred to the term tubule (which suggests a hollow structure) and to the term fibre (which suggests equivalence to hair).