Double immunofluorescence distribution of Fas and keratin in the human thymus. Frozen thymus sections were fixed in acetone and then double-stained with antikeratin (left) and anti-Fas (right) antibodies. After three washes they were stained with rhodamine-coupled antirabbit and FITC-coupled antimouse antibodies. The top plates (×10) show that Fas expression is mainly observed in the medullary area of the human thymus. Control sections (×20) were incubated with the fluorescent conjugates only. The lower plates are enlargements of the framed areas in the upper plates. The white arrows indicate medullary epithelial cells expressing keratin and Fas antigen.