Primed CD4 T cells may develop into effector T cells such as Th1 and Th2, or remain uncommitted as Th primed precursor (Thpp) cells that can subsequently differentiate into Th1 and Th2 cells. Although mouse Thpp-like cells have also been identified among spleen and particularly lymph node cells, farther characterization of these cells has been difficult without a defining cell surface marker. Using Affymetrix GeneChips followed by FACS analysis, we found that in vitro-derived Thpp cells expressed CD73 but not Ly-6A/E, whereas Th1 and Th2 cells showed the reciprocal pattern. CD73+ Ly6A/E- memory CD4 T cells were identified in normal C57BL/6 mice, and the proportion of these cells was highest in lymph nodes, lower in spleens, and lowest in the lungs. . These cells produced IL-2 and MIP-1α, but much less IL-4 and IFN-γ than CD73- Ly6A/E+ cells. Similar results were obtained with additional Ly-6.2 mouse strains, but not Ly-6.1 strains. Restimulation of Thpp-like CD73+ Ly-6A/E- cells in Th1- or Th2-polarizing conditions induced differentiation into popnlations producing mainly IFN-γ or mainly IL-4, respectively. In contrast, the effector-like CB73- Ly-6A/E+ population was more committed, and continued to produce both IL-4 and IFN-γ in both conditions. CD73 and Ly-6A/E expression therefore identify a population of Thpp-like cells in C57BL/6 mice and at least some other Ly-6.2 mice. Copyright © 2005 by The American Association of Immunologists, Inc.