The products of immune lysis, predominantly the stromata, inhibit both guinea pig C′and pig C′, the only ones examined. The components of C′ affected are principally C′2 and C′4; there is a smaller and nonuniform change in C′1 and no measurable change in the activity of C′3. This is not the general pattern observed in the fixation of C′ by antigen-antibody complexes. Rather it is similar to the inactivation of C′ by the dipeptide glycyl-tyrosine, which acts probably as a competitive substrate for proteolytic components of C′. The results clearly indicate that sensitization of stromata with hemolysin is a prerequisite for rendering the stromata inhibitory. The inhibition is sufficiently rapid and extensive to play a part in determining the over-all characteristics of the immune hemolytic reaction.