It has been difficult to define the different factors which contribute to the shaping of the human T-cell receptor (TCR) repertoire. In this study, the influence of the polymorphic human leucocyte antigen (HLA) genes and non-HLA genes on the phenotype of the TCRBV segment repertoire was assessed in a population of HLA class I-matched individuals including three pairs of siblings. The gene expression levels of 24 TCRBV families were evaluated in the CD4+ and CD8+ T-cell subsets of unstimulated peripheral blood mononuclear cells (PBMC) by reverse transcription (RT) and a newly developed competitive polymerase chain reaction (cPCR) assay. Titration experiments demonstrated that the RT-cPCR assay was suitable for an accurate quantification of the relative TCRBV segment expression levels. The T-cell repertoires of HLA-identical siblings were found to be more similar than the repertoires of unrelated individuals. On the other hand, there was no difference in the degree of similarity between the TCRBV repertoires of CD4+ T-cells of HLA class II identical or non-identical unrelated individuals. Furthermore, although most of these individuals had identical HLA class I genes and non-identical HLA class II genes, the TCRBV repertoires of the CD4+ T cells exhibited significantly lower variabilities than the repertoires of the CD8+ T cells. The results of the RT-cPCR assay were supported by flow cytometric analysis of the CD4+ and CD8+ T-cell subsets of the same eight individuals employing 10 different TCRBV segment-specific monoclonal antibodies. These observations argue for a predominant role of non-HLA or non-polymorphic HLA determinants for the shaping of the TCRBV repertoire.