Heterologous anti-epidermal cell serum was obtained by immunizing rabbits with enzymatically dispersed, viable epidermal cells of guinea pigs, followed by absorption of the antiserum with red blood cells (sheep and guinea pig), lymphoid cells and liver powder (guinea pig). Immunofluorescence demonstrated that the antiserum reacted specifically with stratified squamous epithelial cells and thymus epithelial cells including Hassall’s bodies of the guinea pig, monkey and man. It is suggested that the epithelial cells of the skin and thymus have common heteroantigens.