After cloning of laryngeal carcinoma cells from tumor fragments obtained during surgery, the behavior of pure tumor cells was investigated under the inverted microscope. In tissue culture flasks, separately grown tumor cell layers and fibroblast layers were cocultured to visualize the invasion of malignant cells into healthy tissue. Using a time-lapse camera, the cells were recorded. The tumor cells often penetrated the healthy cell formation in a wedge-shaped cell formation. Near the tumor front, an elevated cell motility was found accompanied by continuous changes in the cell shape. Intercellular gaps were broader in the front line than in the more posterior parts of the tumor cell formation. The profound motility of the carcinoma cell enabled selected cells to leave the tumor cell formation completely and to migrate as single cells through healthy tissue. Eventually, some of the single tumor cells migrating through the fibroblast layer moved back to the tumor cell formation. Thus they left healthy appearing tissue behind which was earlier infiltrated by a malignant cell.