To facilitate the detection of invading tumor cells in a three dimensional coculture assay in vitro, the reporter gene Escherichia coli beta-galactosidase (lacZ), was transfected into a human large-cell lung carcinoma cell line GaL23. Multicellular spheroids initiated from the transfected cell line, GaL23LZ, were confronted with fragments of human bronchial tissue differing in their surface composition. While an intact surface epithelium was found to obstruct both adhesion and invasion of tumor cells, an exposed basal lamina augmented adhesion, migration and invasion of tumor cells into the normal tissue. Tumor cells, migrating on the surface of the bronchial fragments, were found to migrate between the epithelial cells and the basal lamina. Fibroblast covered stromal fragments, derived from resected non-small cell lung cancers, were found to be more edible to the invading tumor cells than subepithelial stromal fragments from normal bronchi. The lacZ transfection made it possible to quantitatively analyze the invasive process. While the transfection neither changed the invasive ability of the tumor cells in vitro or in vivo nor their growth pattern in monolayers, three dimensional growth represented by spheroid morphology and clonogenicity in soft agar was significantly changed. This model offers an in vitro system to study qualitative and quantitative aspects of tumor-host relationships in a complex microenvironment which has several similarities to the in vivo situation.