F (fusion) and HANA (hemagglutinin and neuraminidase) glycoproteins of HVJ (Sendai virus) were purified and characterized. The NH2-terminal hydrophobic region of the F1 (larger) subunit of F (fusion)-glycoprotein seems to be required for the hemolytic and cell fusion-inducing activity of the virus for the following reasons. (1) Selective splitting off of a 2,500-3,500 dalton segment from the NH2-terminal region of F1 by chymotrypsin or thermolysin resulted in inactivation of the biological activities of HVJ. (2) At least a part of this region may be exposed to the surrounding medium, since it is preferentially iodinated and is easily split by aminopeptidase M, chymotrypsin, and thermolysin. Tryptic digestion, which does not remove the NH2-terminal region but produce nicking of F1 subunit to subfragments F1a (larger one) and F1b (smaller one), resulted in substantial structural changes evidenced by circular dichroism measurement and iodination by lactoperoxidase method. Trypsin-digested F seems to have the NH2-terminal hydrophobic region buried within hydrophobic interior of the protein (or in the lipid bilayers). Based on these and other results, we propose a hypothesis featuring direct interaction of the hydrophbic region with the lipid bilayers of the target-cell membrane as an important step in fusion reactions between the viral envelope and cell membranes.