During LCMV infection, CD8⁺ T cells expand greatly. Bystander activation has been thought to play a role because few cells score as LCMV specific in limiting dilution analysis. In contrast, we find that at least a quarter of the CD8⁺ cells secrete IFNγ specifically in response to LCMV peptides at the peak of the response. Moreover, by analyzing the expansion of adoptively transferred LCMV-specific, TCR-transgenic CD8⁺ T cells in congenic hosts, we have determined that most of the CD8⁺ cell expansion is virus specific. Analysis of the effect of the monospecific TCR-transgenic T cells on the host response to three LCMV epitopes suggests that CTL precursors compete for sites on the APC in an epitope-specific fashion and that this competition determines the specificity of the response.