Recombinant human gamma 2 chain of laminin-5 was expressed in Escherichia coli, and used to generate specific polyclonal antibodies which were used to study the distribution of the protein in human cancers. A total of 72 biopsies of human cancers were stained, including 23 cases of colon adenocarcinomas, 16 ductal breast carcinomas, 9 malignant melanomas, 14 squamous cell carcinomas of the skin and cervix, and 10 sarcomas. As a control for the specificity of the antibodies, we performed in situ hybridization on adjacent sections of a number of the cases, and in all of these cases the localization of the gamma 2 chain protein and mRNA was identical. We found gamma 2 chain immunoreactivity in cancer cells in all cases of colon adenocarcinomas and squamous cell carcinomas but not in any of the sarcomas, supporting the view that the laminin-5 protein is specific for cells of epithelial origin. Notably, in all of the cases of colon adenocarcinomas, the positive staining was invariably associated with budding cancer cells located at the tip of invading malignant epithelium, whereas the cancer cells deeper in the tumors were most often negative. The staining was cytoplasmic in all cases and only in one case did we see additional extracellular immunoreactivity, indicating that this laminin isoform in cancer tissue is not laid down in the extracellular matrix but probably exerts its function at the cell surface or in its immediate vicinity. Using in situ hybridization to analyze the coexpression of laminin-5 and components of the plasminogen activation system, we found that the histological distribution of laminin-5-positive budding cancer cells at the invasion front in colon adenocarcinomas was identical to that of the receptor for urokinase-type plasminogen activator. These findings suggest that laminin-5 is a marker of invading cancer cells in at least some human malignancies, and that it therefore might represent a valuable marker for the invasive potential of these cancers. The colocalization of laminin-5 and urokinase-type plasminogen activator receptor in a subset of cancer cells in colon cancer also suggests that a controlled up-regulation of a number of gene products is a characteristic of budding colon cancer cells, and that these gene products serve functions crucial for the invasive phenotype of these cancer cells.