In an attempt to elucidate the mechanism underlying the formation of cutaneous horns, 43 cases of cutaneous horns were examined histologically and histochemically. Three of 43 cases were also investigated by using direct immunofluorescence technique and electron microscopy. The structure identical to "Hornmark" was found in all of 43 cases of cutaneous horns except two which consisted only of horny masses. The substance found in the intercellular space of "Hornmark" was homogenous, eosinophilic and mostly diastase-resistant PAS positive, and showed strongly positive immunofluorescence for immunoglobulins, complement, and fibrinogen. Electron microscopically, it was moderately electron dense, fine granular and in part fine fibrillar, and was similar to the contents of capillary lumen. From these findings, it was suggested that the substance found in "Hornmark" consisted mainly of the components of plasma protein, and that coagulated plasma protein might play an important role on the increased cohesiveness of the horny cells, i.d. on the formation of cutaneous horns.