The treatment of antigen-antibody complexes with cysteine-HCl results in the splitting of these complexes and antigen liberation. Complexes with IgM antibodies are 4–8 times more resistant to cysteine than complexes with IgG antibodies. Dissociation of the immune complex is associated with inactivation of the antibody. It is found that antibodies bound to antigen are more sensitive to cysteine than unbound antibodies: unbound IgM is inactivated only by 31.4 and 15.7 mg/ml cysteine; unbound IgG is not sensitive to these concentrations of cysteine. For inactivation of IgM and IgG antibodies complexed with the antigen, 3.75 and 0.45 mg/ml cysteine-HCl is required, respectively Other reducing agents (glutathion and sodium sulfite) were found to be effective in dissociation of immune complexes too.