Gastrointestinal adenocarcinoma-derived cell lines were studied in order to determine their pattern of expression of basement membrane components and their ability to form a basement membrane. In contrast to the well-preserved expression of laminin beta 2, beta 3, gamma 1, and gamma 2 chain mRNAs, five of eight gastrointestinal cancer cells lacked alpha 3 mRNA. Immunohistochemical and electron microscopic examination of four cell lines transplanted subcutaneously to SCID mice demonstrated the presence of both alpha 3 and alpha 5 chains and the formation of a basal lamina in two cases. The other two cell lines lacked both alpha 3 and alpha 5 chains and could not form a basal lamina, suggesting that this deficiency may be a factor which affects their ability to form a basement membrane. This abnormality might play some role in stromal invasion by tumour cells in gastrointestinal cancer.