To the Editor: In the course of an extensive investigation of the toxoplasma-antibody activator system (Sabin AB, Feldman HA: Dyes as microchemical indicators of a new immunity phenomenon affecting a protozoon parasite [toxoplasma]. Science 108:660–663, 1948) we found that human serums deficient in C5, C6, C7 or C8 (serums deficient in C5 and C6 were supplied by Dr. John P Leddy, and serum deficient in C8 by Dr. Bruce Petersen) lack such “activator” activity. The addition of C7 to the C7-deficient serum immediately restored its “activator” system to full activity. Antibodies indicative of previous infection were not detected in this. . . No extract is available for articles shorter than 400 words.