The Notch gene family encodes transmembrane proteins that have been implicated in control of diverse cellular differentiation events in the fly, frog, and mouse. Mammalian Notch1 is expressed at high levels in thymus and is mutated in a subset of human T-cell acute lymphoblastic neoplasms, suggesting a role in T-cell differentiation. To investigate the patterns of expression of NOTCH1 protein in thymocytes of the developing and mature thymus, antibodies raised against NOTCH1 were used to perform immunohistochemical and flow cytometric analyses. Strong staining for NOTCH1 within the fetal murine thymus was observed as early as 13.5 days postcoitum. By 17.5 days postcoitum, preferential staining of superficial cortical thymocytes was observed, with weak staining of developing medulla. Flow cytometric analysis and immunohistochemical staining of flow-sorted cells confirmed that the highest levels of NOTCH1 expression in adult murine thymus were present in immature cortical thymocytes (CD24high, CD4-CD8-). In contrast, NOTCH1 expression was low or absent in more mature cortical thymocytes (CD24low, CD4+CD8+), whereas intermediate levels of expression were observed in CD4+CD8- and CD4-CD8+ cells. These data indicate a dynamic pattern of NOTCH1 expression during T-cell differentiation and suggest that downregulation of NOTCH1 may be required for maturation of cortical thymocytes.