['To characterize the effect of homotypic cell-to-cell collisions upon the motile activities of two rat prostatic cancer cell lines of markedly different metastatic potential.', 'The movements of strongly and weakly metastatic MAT-LyLu and AT-2 cells, respectively, were recorded under an inverted microscope at 37 degrees C. The motile activities of the cells at various cell densities were characterized quantitatively by computer-aided tracking methods and image analysis. The following variables were assessed: speed of movement, final displacement, coefficient of movement efficiency, diffusion constant and positive flow.', 'MAT-LyLu and AT-2 cells showed only limited motile activity in sparse cultures where there was little contact amongst the cells. However, under these and all other subsequent conditions tested, the motile activity of the MAT-LyLu cells was higher than the AT-2 cells. As the density of the cultured cells was increased (leading to more cell-to-cell contacts) there was a significant increase in motility. This effect was more pronounced for the AT-2 than for the MAT-LyLu cells, resulting in visible acceleration of movement by direct physical contact among the colliding cells. The motile activities of the tumour cells was only slightly affected by conditioned media.'] Do homotypic cell-cell contacts stimulate the motile activity of rat prostate cancer cells?