A 76-year-old man was found to have low (33% normal) serum complement (C) hemolytic activity, although C3 and C4 protein levels were normal. Further evaluation of his serum and plasma indicated that all C components were present in normal or elevated amounts except for C9, which was undetectable by both antigenic and functional assays. Addition of purified human C9 led to full restoration of the hemolytic activity. Family studies demonstrated that the deficiency was inherited as an autosomal codominant trait and was not linked with alleles at the HLA-A or HLA-B loci. The patient had no history of recurrent or unusual infections and no evidence of autoimmune disease. The availability of serum totally lacking in C9 permitted an investigation of the lytic capacity of the C5b-8 segment of the C attack mechanism, which was pursued in kinetic studies on the hemolysis of erythrocyte intermediates. These studies indicated that hemolysis occurred approximately 100 times slower in patient than in normal serum, using either EA or EAC1-7 intermediates as target cells. Serum bactericidal activity also was slower in patient serum, occurring at a rate about 1/35 that observed in normal serum. These studies provide direct independent evidence that cytolysis of erythrocytes and bacteria can be mediated by C5b-8, and allow a quantitative estimation of the increment in the rates of these reactions provided by normal serum levels of C9. The presence of readily detectable though slow hemolytic activity of C9-deficient serum may account for the difficulty in identifying individuals with this defect.