Neither the hemolytic activity nor the protein level of the seventh component of serum complement (C7) was detectable in an 8‐year‐old girl with nephritis, but in her parents and her brother, they were about half of the normal level. The patient was a homozygote type with a complete deficiency of C7 while her parents and brother were all heterozygote type with a partial deficiency of C7. C7consuming activity was demonstrated in the native serum of the patient with complete C7 deficiency, and it was found that large amounts of C56were readily generated upon incubation of the patient's serum with zymosan. It is proposed that the C7consuming activity in the native serum of this patient is due to small amounts of C56generated during the activation of serum complement by some kind of infection such as a common c