Lipid vesicles carrying the purified membrane C5b-9 complex [C5b-9(m)] of complement were analyzed immunochemically and in the electron microscope after treatment with a combination of trypsin and alpha-chymotrypsin. Under reducing conditions, the externally oriented annulus was removed. The remaining part of the C5b-9(m), representing approximately half of the total mass of the macromolecular complex, was visualized in the electron microscope as a hollow cylindrical structure with walls of 1-nm thickness. This structure remained tenaciously attached to the lipid bilayer, projecting 8-9 nm from the external membrane surface into the aqueous environment. Cleavage of C5b-9(m) by proteolysis and reduction resulted in a sharp reduction of tis antigenic determinants. One hydrophilic protease-resistant C5 derivative was released from the membrane and recovered in the fluid phase. The membrane-bound residue almost totally lacked antigens precipitable with antisera to C5, C6, C9, and C5b-9(m).