Serum samples from 1,258 normal adult blood donors were assayed for complement activity. The average complement titer for the entire sample was 38.14. There was no difference in complement activity with respect to sex, blood group or Rh***(D) type. The level of circulating complement appears to be higher in older humans and in heavier individuals.In this kind of experimentation, the need for a standard complement for assay of different batches of sheep erythrocytes is emphasized.