Surface protein patterns of 12 various tumorigenic and non-tumorigenic liver epithelial cell lines (IAR series) were studied by means of lactoperoxidase catalyzed iodination followed by the gradient sodium dodecyl sulfate polyacrylamide gel electrophoresis. These patterns were found to be very similar except a protein with an apparent mol. wt. about 150 kilodaltons which appeared in cells of tumors derived from in vitro transformed epithelial liver cells. Surface fibronectin was present in all the three non-tumorigenic lines, and in 7 of 9 tumorigenic cell lines examined. The indirect immunofluorescence revealed that in non-tumorigenic lines this protein was localized predominantly in the regions of cell-to-cell contacts, while in tumorigenic cells fibronectin often formed fibrillar networks similar to those of fibroblasts.