The activation of subcomponents C1r and C1s in the first component of complement, C1, when bound to antibody-antigen complexes was investigated. Activation was followed both by the splitting of the peptide chains of subcomponents C1r and C1s and by the development of proteolytic activity. For the maximum rate of activation to occur, all components must be present in approximate molar proportions of antibody: C1q:C1r:C1s of 13:1:5:5. For activation of subcomponent C1s, subcomponents C1r or C1r, but not C1r inactivated with iPr2P-F (di-isopropyl phosphorofluorideate), are effective. For activation of subcomponent C1r, subcomponents C1s, C1s or C1s inactivated with iPr2P-F are effective. Subcomponent C1s is activated by C1r, and C1r is activated autocatalytically, probably through the formation of an intermediary C1r. in which the peptide chain is unsplit but a conformational change caused by interaction with the other components has led to the formation of a catalytic site able to split subcomponent C1r to C1r.