The ninth component of complement (C9) polymerizes to tubules either spontaneously at a very slow rate (3 days at 37 degrees C) or within minutes if induced by C5b-8 as part of the membrane attack assembly (Tschopp, J., Podack, E. R., and Müller-Eberhard, H. J. (1984) J. Immunol., in press). The spontaneous formation of tubular sodium dodecyl sulfate-resistant C9 polymers was completely inhibited in the presence of 1 mM EDTA. In contrast, the presence of Cu2+, Zn2+, and Cd2+ accelerated C9 polymerization. Zn2+ was the most efficient metal ion. At a molar ratio of one Zn2+ per monomeric C9, all C9 polymerized within 2 h. At Zn2+ concentrations above 10(-3) M, tubular C9 formation was inhibited due to nonspecific aggregation of C9. It is concluded that the presence of metal ions is required for the formation of tubular poly C9 and that Zn2+ in particular catalyzes C9 polymerization.