We report the first direct observation of chemotaxis in slowly moving malignant cells. Two sarcoma cell lines of different metastatic potential were used. In a direct-viewing chemotaxis chamber with two concentric wells, the pooled trajectories of highly malignant rat T15 cells were strongly biased toward the outer well which contained platelet-derived and insulin-like growth factors. In individual experiments, however, the trajectories of the T15 cells showed a significant directional bias which, depending on the cell distribution, sometimes deviated by as much as 170 degrees from the gradient direction. Cells of a less malignant rat line, K2, did not respond to the gradient but a strong K2 response appeared if T15 cells were placed in the outer well along with the growth factors. We conclude that stimulated T15 cells release a chemoattractant, for both T15 and K2 cells, which overrides any chemoattractive effect of the growth factors. These results call into question whether growth factors are ever directly chemotactic in this system and demonstrate the need for direct observation in determining whether any substance is a direct chemoattractant.