Naive CD4(+) T cell populations that express TCRs specific for different foreign peptide-MHC class II complex (pMHCII) ligands can vary in size over several orders of magnitude. This variation may explain why immune responses to some peptides are stronger than others. In this study, we used a sensitive pMHCII-tetramer-based cell enrichment method to study the derivation of two naive foreign pMHCII-specific naive CD4(+) T cell populations that differed in size by 8-fold in normal mice. Analysis of mice in which thymic negative selection was impaired revealed that the smaller population underwent more clonal deletion than the larger population. In addition, large naive cell populations tended to recognize peptides with tryptophan residues as TCR contacts. Thus, the foreign pMHCII that tend to be recognized by large naive populations induce minimal clonal deletion and contain certain amino acids with the capacity to interact favorably with TCRs.