Motility factor produced by malignant glioma cells: role in tumor invasion. To better understand the cellular mechanism of tumor invasion, the production of a cell motility-stimulating factor by malignant glioma cells was studied in vitro. Serum-free conditioned media from cultures of rat C6 and human T98G cell lines contained a factor that stimulated the locomotion of the producer cells. This factor was termed the "glioma-derived motility factor." The glioma-derived motility factor is a heat-labile protein with a molecular weight greater than 10 kD and has relative stability to acid. The factor showed not only chemotactic activity but also chemokinetic (stimulated random locomotion) activity in the two types of glioma cells studied. Although glioma-derived motility factors in conditioned media obtained from two different cell origins are likely to be the same, chemokinetic migration of T98G cells to their conditioned medium was much stronger than that of C6 cells to theirs. Coincubation of cells with cytochalasin B, which disrupts the assembly of cellular actin microfilaments, almost completely inhibited the cell migration stimulated by glioma-derived motility factor. Cytochalasin B also induced marked alterations in cell morphology, including cell retraction and arborization, while the drug did not affect cell attachment to culture dishes. These results indicate that glioma cells produce a motility factor which may play a role particularly when tumor cells are detached and migrate away from the original tumor mass, thus promoting tumor invasion. Also, glioma cell migration stimulated by the motility factor requires the normal organization of cytoskeletons such as actin microfilaments. 