The ability of cells to organize collagen fibrils is fundamental to a variety of processes found in embryogenesis, wound healing, fibrosis, and scar formation. We previously isolated a monoclonal antibody (mAb A3A5) which inhibits human fibroblast‐mediated collagen gel contraction, an in vitro model producing the process of collagen morphogenesis. Human fibronectin (FN) has been shown to be the antigen of A3A5. The present study aimed at identifying the A3A5 epitope to reveal the mode of binding between collagen, FN, and fibroblasts in the process of gel contraction. The epitope was sought in FN fragments obtained by pepsin digestion and in recombinant FN fragments expressed in Escherichia coli by determining their immunological reactivity with A3A5, and was identified as a short segment consisting of the fourth through the amino half of the fifth FN type III. We propose a new functional domain of FN which plays a crucial role in the binding of fibroblasts to collagen fibrils and is involved in collagen morphogenesis.