We have found that the promoting effects of proteins from fibroblast conditioned medium (PCM) promote adhesion of rat liver epithelial cells (nonmalignant RL-34 cell line) seeded on collagen coated substratum. Since the effect was completely inhibited by treating the cells with cycloheximide the protein(s) are probably produced by the RL-34 cells. We also found that PCM-A fraction which is identical to fibronectin purified from whole PCM had no effect on RL-34 cell adhesion and that the adhesion was not inhibited even by adding affinity purified anti-FN antibody to the medium containing FN-free PCM (PCM-B fraction). This indicates that adhesion of rat liver epithelial cells on collagen substratum does not require FN, but requires the adhesion promoting factor(s) existing in PCM-B.