AbstractAn antiserum raised against human fetal and childhood thymocytes (anti‐Thy) and absorbed with peripheral lymphocytes (tonsil) detected an antigen(s) shared by thymocytes, T cell‐acute leukemias, activated peripheral T cells and a subset of peripheral T cells. The antigen was expressed by the negative circulating T cell subset after mitogen activation of that separated population. The antigen was shown to be separate from the E rosette receptor, another anti‐T cell serum detected antigen, and β2‐microglobulin; its expression was not related to a particular phase of the cell cycle. The results suggest that the antigen is expressed by T cells only under certain maturational and proliferative co