Monoclonal antibodies were used to probe the cell surface of organ-selected metastatic variant cells. Previously we defined a liver metastasis-associated antigen (LMAA) by means of the reaction of a monoclonal antibody with a liver metastasis selected tumour variant cell line, MDCC-AL2. The monoclonal anti-LMAA antibodies specifically inhibit liver metastasis of AL2. There is a correlated, clonal variation in LMAA expression and liver metastasis in both the AL2 cell line and an overy-selected metastatic variant MDCC-AL3. The variation in liver metaatatic ability is thought to represent clonal progression of the tumour cell lines. The LMAA probably represents only one of the ways in which a tumour cell may give rise to a live metastasis. Two hypotheses are discussed utilizing the LMAA in a functional role in the specific trapping of metastatic tumour cells in the liver or the successful colonization of the liver by metastatic tumour cells.