The src family tyrosine kinase, Lck, transduces signals from the pre-TCR complex which regulate the development and expansion of CD4/CD8 double-positive (DP) thymocytes from CD25+ CD4/CD8 double-negative progenitors. We and others have recently shown that sublethal γirradiation bypasses the need for TCRβ expression to promote the development and expansion of DP thymocytes in acid or recombinase-activating gene (RAG)-deficient mice. Here we demonstrate that γ-irradiation activates an Lck-dependent signaling process in Immature thymocytes similar to that initiated physiologically by the pre-TCR complex.