Results of recent studies indicate that some cultured human carcinoma cell lines are capable of proliferating autonomously in serum-free medium as a result of the synthesis and secretion of transforming growth factor alpha (TGF alpha). TGF alpha interacts with epidermal growth factor receptor (EGFR) and induces its activation. In an attempt to extend these observations, we evaluated TGF alpha-mediated autonomous growth and constitutive EGFR activation in the human adenocarcinoma cell line SW403. The cell line shows synthesis of EGF receptors and TGF alpha but not EGF, and exhibits constitutive phosphorylation of the 170-kDa EGFR. Use of blocking anti-EGFR monoclonal antibodies (mAb) inhibits autonomous growth of SW403 cells and leads to a significant reduction of receptor phosphorylation. The inhibitory effect of the blocking anti-EGFR mAb is reversible upon addition of TGF alpha. In contrast, autonomous proliferation of SW403 cells is not inhibited by addition of neutralizing anti-EGF mAb. Our findings suggest that the proliferation of cells of the human SW403 adenocarcinoma cell line is regulated by an autocrine TGF alpha loop and that this regulatory pathway can be interrupted by using anti-EGFR mAb.