An effective molecule titration for the ninth component of complement in the biologic fluids of man was developed using EAC1-8 cells produced by treating EAC14 cells with a chromatographic fraction of human serum containing C2, C3, C5, C6, C7, and C8. Kinetic studies of the interaction of EAC1-8 with C9 indicated that this component was depleted from the fluid phase, and that the lytic reaction proceeded most rapidly at ionic strength 0.145, and at a temperature of 37°C. The mean value for C9 in normal serum was 52,000 ±12,000 units/ml. The mean serum C9 for patients with DJD, rheumatoid arthritis, or SLE without active renal disease was approximately twice the mean for normal individuals. Patients with SLE and active renal disease had a mean C9 value which fell within the normal range, but was significantly lower than in patients with SLE who did not have active renal disease. Two instances of absolutely subnormal C9 levels were observed in patients during attacks of florid SLE, including nephritis. Since the usual change in serum C9 in rheumatic diseases is a marked elevation, the occurrence of a subnormal value reflects circumstances in which depletion due to activation of the sequence exceeds the increases associated with the inflammatory response.