T cell receptor (TCR) transgenic thymocytes specific for the LCMV gp peptide are normally positively selected to the CD8 lineage. Transgenic thymocyte development was substantially reduced in the absence of these CD8 coreceptors. However, efficient positive selection was restored when TCR transgenic CD8-/- fetal thymic lobes were cultured with a peptide variant of the wild-type ligand. These mature thymocytes were functional, as shown by their ability to respond against strong peptide agonists. Additional experiments demonstrated that transgenic positive selection was peptide-specific. These results prove that CD8 does not possess essential signaling properties that are necessary for T cell development. In addition, the unilateral commitment of transgenic thymocytes to mature CD4-TCR(hi) T cells expressing intracellular perforin suggests that there must be some instructive component to CD4 down-regulation and lineage commitment during thymocyte selection.