Formation of rosettes with sheep red blood cells (SRBC) by peripheral blood lymphocytes and thymocytes has been studied. Trypsinization of these cells abolishes the capacity to form SRBC-rosettes. The process of re-establishment of this capacity to form SRBC-rosettes, in vitro, has been shown by use of metabolic inhibitors to require transcription, translation, and protein synthesis. These observations are interpreted in terms of the T cell markers as the expression of a definitive process or step in lymphoid cell differentiation.