Experiments are described which measure the effect of ionic strength on C′ fixation. It has been demonstrated that C′ fixation by both protein and polysaccharide immune systems is enhanced by NaCl concentrations less than and inhibited by NaCl concentrations greater than 0.15 M. This enhancement or inhibition of C′ fixation by changes in NaCl concentrations appears to affect a reaction step subsequent to antigen-antibody aggregation and prior to C′3 participation.