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There is accumulating evidence that the micro-environment plays a pivotal role in the progression of malignancy and that key stromal changes can be identified which influence this progress. This investigation focussed on two specific areas; fibroblast phenotype and the composition of the extracellular matrix, especially regarding the protein-tenascin-C (TN-C).;The heterogeneity within colonic fibroblast populations is becoming increasingly apparent. Using immunohistochemical techniques this study defined specific populations and identified consistent phenotypic changes in these cells in malignancy. In particular CD34 expression was lost in sub-mucosal fibroblast and alpha smooth muscle actin (alphaSMA) expression increased within tumour stroma. There was also evidence of progressive loss of high molecular weight caldesmon (hCD) expression by the pericryptal myofibroblasts with the development of malignancy. The stromal changes were specific to malignancy and were not demonstrated in pre-invasive adenomas.;TN-C is an important component of the extracellular matrix (ECM) which can occur as several different isoforms. Immunohistochemical techniques were used to demonstrate alterations in the distribution of TN-C between normal and malignant colon and the presence of isoforms containing exon 14 in both. The distribution of TN-C in pre-invasive adenomas was the same as that in normal colon but some sub-mucosal TN-C expression including exon 14 containing isoforms was observed in acute inflammation. Polymerase chain reactions (PCR) identified a total of 7 TN-C isoforms in colonic tissue with no obvious association between isoforms profile and disease.;TN-C can also be expressed by tumour cells and PCR demonstrated multiple isoforms in the colorectal tumour cell lines SW480, SW620, HCT15 and HT29. However, TN-C expression did not appear to correlate with invasive capacity as demonstrated in preliminary in-vitro invasion assays.