Human C9 treated with trypsin is initially cleaved into two fragments with relative mol. wts of 53,000 and 20,000. This limited cleavage of C9 induces a 2.4-times increase in the hemolytic activity of C9 when compared to untreated C9. This difference diminishes when C9 activity is tested in an assay using a prolonged incubation time of C9 with C5b-8-bearing red blood cells. Trypsinization of C9 also promotes spontaneous C9 polymerization. SDS-resistant tubular C9 complexes are formed at a C9 concn of 1 mg/ml within 8 hr at 37 degrees C. Our data indicate that specific limited proteolysis of C9 not only induces spontaneous C9 polymerization but also increases the hemolytic activity of C9, suggesting that a similar molecular mechanism is involved in both processes.