For colorectal carcinomas, there is evidence that marked discontinuity of the epithelial basement membrane (EBM) is associated with higher malignant potential. Since the metastatic process appears to be selective, more discontinuous EBMs might be expected in secondary rather than in primary tumours. To test this prediction, we examined a series of 60 cases of colorectal carcinoma for which samples of lymph-node or liver metastases were available. Sections were immunocytochemically stained for laminin, and the continuity of tumour EBM was then assessed by observational rating as well as by detailed morphometric analysis for a sample of cases. Contrary to the above prediction, we find that EBMs tend to be more continuous in secondary tumours than in corresponding primary tumours. These results could be explained by the possibility that local tissue environmental factors have a major influence on EBM continuity. Supporting evidence comes from our previous observation that EBM is very discontinuous at the advancing edge of primary colorectal carcinomas, where the tumour is adjacent to collagen-I-deficient stroma. Further evidence from the present study is that the EBM is extremely discontinuous at the interface between metastases and specialised parechymal tissue, but more continuous at the interface between metastases and stromal connective tissue. Since basement membranes affect the differentiation and behaviour of adjacent cells, these findings suggest that host tissue may influence invasive activity through their effects on EBM continuity.