Objectives : To analyze the effect of retinoids on the differentiation in HNSCC xenografts. Materials and Methods : RA (20mg/kg) or 13-cis-RA (60mg/kg) was orally administered once in a day for 30 days in the xenograft model we prepared using athymic nude mice with AMCHN-4 and -6. We carried out H & E staining and immunohistochemical staining with the monoclonal antibody against involucrin and cytokeratin 10. Results : Both RA and 13-cis-RA were found to suppress the differentiation of AMC-HN-4. Interestingly, RA enhanced the differentiation of AMC-HN-6, although 13-cis RA did not exhibit any effect on the differentiation. These results suggest that in vivo effect of retinoids on the HNSCC growth and differentiation might be various. Retinoids-induced P450 in AMC-HN-6 might be one of the mechanisms to explain the reason why the retinoids exhibit various functions in the HNSCC.