CD4 contributes to antigen recognition of T cells by binding to class II MHC molecules. There is heterogeneity in expression of CD4 coreceptor among CD4+CD8+ thymocytes. We have investigated whether the expression level of coreceptor influences positive selection. Thymocytes of mice expressing transgenic lambda2(315)-Ig-light-chain/I-Ed specific TCR are poorly positively selected because they fail to allelically exclude endogenous TCR alpha chain genes and because there is no skewing towards CD4. Transient overexpression of CD4 during thymocyte development, in mice transgenic for both TCR and CD4, resulted in skewing towards CD4 in the periphery, reduced rearrangement and expression of endogenous alpha-chains, and decreased levels of thymocyte RAG-1 transcripts. Kinetic BrdU labeling experiments showed that single CD4+ thymocytes developed faster, representing the predominant population even in the cortex of the double transgenic thymi. These results demonstrate that increased coreceptor expression can compensate for poorly selectable TCR, supporting avidity and instructional models for positive selection of thymocytes.