Regulatory factor X (RFX) is an essential MHC class II transcription factor and contains three distinct subunits of which RFX-B is one. Aberrant expression of MHC class II genes is associated with autoimmunity, tumour growth and failure to mount an immune response. RFX-B protein expression in human colorectal adenocarcinomas and in normal adjacent tissue was analysed in this study. Western blot analysis showed a suppression of nuclear RFX-B protein in the tumour tissue. Immunohistochemistry revealed that the RFX-B protein levels in macrophages were generally lower in colorectal cancerous tissue compared to adjacent non-cancerous tissue and that focally and not frequently tumour and normal epithelial cells were stained weakly for RFX-B. As the expression of MHC class II correlates with the intensity of the immune response system these results may support the idea that cancer is associated with immunodeficiency and that low levels of RFX-B in interstitial macrophages could partly explain this thesis.