The presence and nature of interpeptide disulfide bonds in HANA (hemagglutinin and neuraminidase) glycoprotein and F (fusion) glycoprotein of HVJ [hemagglutinating virus of Japan] (Sendai virus) are described. In the case of HANA, subunits of the same or very similar MW were interconnected with a disulfide bond(s). Cleavage of the bond(s) can easily be achieved by the addition of 1 mM dithiothreitol with concomitant loss of the biological activities of the glycoprotein. After splitting of the interconnecting bonds, all the HANA protein subunits remained bound on the viral membrane. To observe the cleavage of the interpeptide disulfide bond between the F1 and F2 subunits of F glycoprotein, higher concentrations of sulfhydryl compounds were required than were necessary for HANA protein. Splitting of the disulfide bond under either denaturing or nondenaturing conditions failed to release both segments of F protein from the virion. Therefore, F glycoprotein seems to have at least 2 membrane binding sites, one on F1 and the other on F2. The disulfide bond which connects the HA1 and HA2 subunits of influenza virus is slightly cleaved under non-denaturing conditions. Addition of 8 M urea or 6 M guanidine HCl, which completely inactivates HA activity, was necessary for the splitting of this disulfide bond by thiol compounds. The HA1 subunit was released from the virion after the cleavage. Thus, unlike F1 and F2 of HVJ, the HA1 subunit seems to have no hydrophobic binding site to the membrane. A model for the arrangement of these subunits on the viral membrane is proposed.