The invasion and migration occurring in primary neoplastic tissue explants were studied by using a three-dimensional collagen matrix model, subsequent time-lapse videomicroscopy, and computer-assisted cell tracking. We show that not only single cells but groups of clustered cells comprising 5 to more than 100 cells detach from the primary tumor lesion and migrate within the adjacent extracellular matrix. These clusters were highly polarized, resulting in a high directional persistence of migration. Locomoting cell clusters were observed in primary cultures from invasive oral squamous cell carcinomas (6 of 9), ductal breast carcinomas (2 of 3), and rhabdomyosarcoma (1 of 1), whereas normal oral mucosa (0 of 4) was cell cluster negative. Thus, locomoting cell clusters could be a novel and potentially important mechanism of cancer cell invasion and metastasis.