Primary human colorectal carcinomas which had already metastasized to the draining lymph nodes have been studied by transmission electron microscopy and tissue culture. The invasive edges were identified by examining mirror-image blocks for light and electron microscopy. In the invasive edge there are single cells and small clusters of cells which have ultrastructural features suggestive of active cell movement; clusters of cells are found in the lymphatic trunks and in the subcapsular sinus of draining lymph nodes. Small clusters of cells survive in short-term culture; some of these cells are actively motile. These findings support the view that there is a sub-population of motile and invasive cells at the edge of human colorectal cancers.