Many aspects of cell social behavior, including aspects of tumor invasiveness, embryonic development, and wound healing, can be explained by the principle of contact inhibition (CI) of cell movement. CI refers to the tendency of fibroblasts cultured on a plane substratum to cease movement on contacting other fibroblasts. A problem in studying collisions between cells on a flat substratum is that it is difficult to control the specific regions of the cell that come in contact. In this study we used grooved micromachined titanium substrata to produce collisions between the following cell combinations: fibroblast/fibroblast, fibroblast/epithelium, and epithelium/epithelium. The cells were oriented by the substratum so that the leading lamellae of the cells confronted each other. Cell behaviors before and after contact were observed and recorded using time-lapse cinemicrography employing Nomarski reflected light differential interference microscopy. Electron-microscopy sections were prepared from areas where cell interactions occurred. Fibroblasts (F) moved significantly faster and more persistently on grooved than on smooth surfaces, but the speed of epithelial (E)-cell locomotion was not significantly altered. The grooves, however, guided the direction of locomotion for both cell types. When cultured on grooved surfaces in such a manner that the F and E cells collided head-on, the F, but not the E cells, frequently demonstrated contact inhibition of movement. However, after such collisions, significantly more F continued to invade the E sheet than were observed after F-E collisions on smooth surfaces. After F-F collisions on grooved surfaces, most cells moved to the sides of the grooves and continued in their original directions, while on smooth surfaces they moved off in various different directions. A possible explanation of these observations is that a grooved surface produces and maintains F polarity so that the direction of locomotion is less readily altered by cell-cell interactions.