In this study, we examined expression of the major histocompatibility complex (MHC) class I molecules and of the mRNAs for MHC class I-mediated antigen presentation machinery molecules in highly purified C57BL/6 mouse testicular germ cells. Although H2 class I molecules were not detected on the cell surface, small amounts of the molecules were detected inside the cells by flow cytometric and immunoprecipitation analyses. Interferon (IFN)γ treatment did not affect expression of the molecules on the surface by flow cytometric analysis. In addition, expression of mRNAs for MHC class I-mediated antigen presentation machinery molecules in the germ cells was analyzed by reverse transcription-polymerase chain reaction analysis. The faint constitutive expression of mRNAs for H2-K, β₂-microglobulin (β₂m), tapasin, and calnexin was observed in the cells. IFNγ treatment up-regulated expression of mRNAs for H2-K, β₂m, TAP1 (a peptide transporter), and LMP2 (a proteosomal subunit). These findings indicate that impaired expression of MHC class I molecules on the cell surface of mouse testicular germ cells is mainly caused by the post-transcriptional regulatory mechanism.