1. 1. [ 3H]histidine was injected into newborn rats and 48 h later, protein constituents were isolated from cornified cells of the epidermis. Two radioactive fractions obtained (Fractions II and III) were analyzed by Sephadex column chromatography and gel electrophoresis. 2. 2. Fraction II contained a highly radioactive peak, F IIc, consisting of a radioactive major component with a molecular weight of 21 000 and non-radioactive minor components. 3. 3. Fraction III appeared to be fibrous in nature and contained a large nonradioactive first peak, F IIIa, and a small radioactive peak, F IIIb. F IIIa consisted of two components of mol. wt 68 100 and 61 000, while F IIIb showed a major component of mol. wt 57 000 and several minor components. 4. 4. Attempts to produce antisera in rabbit directed to these protein fractions resulted in formation of antibodies against F IIIa and the non-radioactive component of F IIc. Direct immunofluorescence microscopy demonstrated that F IIIa and F IIc are present in cornified cells as well as other parts of the epidermis, but not in the hair follicles, dermis, or liver of newborn rats. 5. 5. The present study confirms the autoradiographic studies, which demonstrated that proteins synthesized in granular cells as a result of [ 3H]histidine incorporation become constituents of cornified cells. However, it was not possible to determine the localization of the radioactive proteins in granular cells by the techniques used.