By means of an immunocytochemical method, using an antiserum directed against Thymus Factor X (TFX) the presence of TFX-cross-reactive antigen on the surface of rat thymocytes was demonstrated. In the electron microscope, the product of the immunocytochemical reaction was localized as a fine granular and electron opaque material on the surface of some medium-size and small thymocytes. It was shown that the intensity and distribution pattern of the reaction product on the surface of the thymocytes varied markedly. Preincubation of thymocytes with TFX resulted in more intense labelling. The presence of TFX and cross-reactive native thymic antigens on the surface of rat thymocytes as revealed by anti-TFX serum, allows us to suppose that these thymic proteins are bound to surface of the cells by specific surface receptors. © 1979 Springer-Verlag.
