Recent studies have shown that TCR β chain expression can effect the differentiation of CD4−CD8− double-negative (DN) thymocytes to CD4+CD8+ double-positive (DP) thymocytes. The TCR β chain is expressed on the surface of DP thymocytes in association with CD3γ, δ and ε chains, suggesting a potential role for CD3 components in this signaling process. We now report detection of a very tow level of surface expression of CD3ε on adult DN RAG-2−/−; thymocytes. This surface CD3ε was associated with CD3γ and δ chains, as detected by anti-CD3ε immunoprecipitation analyses. Significantly, injection of anti-CD3ε mAb into RAG-2−/− mice led to the accumulation of an IL-2Rα− CD2+ DP cell population and a nearly 100-fold increase in thymic cellularity to essentially normal levels. Together, these data strongly indicate that TCR β chain-mediated developmental signals are transduced by CD3 components and provide potential insights into mechanisms by which TCR β chain expression may effect this process.