An IgG1 monoclonal antibody, FDC-6, was established, which defines a unique fibronectin (FN) domain, located between the "Hep-2" and the "Fib-2" domains, in the COOH-terminal region of FNs isolated from hepatoma, sarcoma, and fetal fibroblasts. A systematic study with this antibody indicates the presence of two classes of human FNs. (i) FN from fetal connective tissue, placenta, amniotic fluid, hepatoma, and colon carcinoma as well as cell lines from fetal tissues (WI-38), hepatomas (HuH-6 and HuH-7), and sarcoma (VA13) was characterized by the presence of the FDC-6-defined domain and by a high molecular weight (subunit Mr, 310,000-335,000). (ii) In contrast, FN from normal adult tissues and plasma was characterized by a lower molecular weight (subunit Mr, 285,000-295,000) and lack of reactivity with FDC-6 and is therefore devoid of the FDC-6-defined domain. The FDC-6-defined domain is therefore called the "oncofetal" domain, and FN containing this domain is hereby called "oncofetal FN" (onf FN). The onf FN is similar to the previously known "cellular-form" FN. FN from normal adult tissues and plasma, lacking the oncofetal domain, is hereby called "normal FN" (nor FN). The nor FN is similar to the previously known "plasma-form" FN. Development of FN from fetal to adult form is associated with loss of the oncofetal domain defined by the FDC-6 antibody, and oncogenic transformation is associated with activation in synthesis of the oncofetal domain defined by the FDC-6 antibody.