Ca(++) potentiates the inactivation of human complement by streptokinase-activated plasmin and chloroform-activated bovine plasmin. The optimum concentration of Ca(++) varies between 10(-2)M and 10(-3)M, decreasing with increasing concentrations of streptokinase. The susceptibility of the components of complement to inactivation by plasmin is different in the presence and absence of Ca(++). C'2 and C'4 are most readily inactivated by plasmin in the presence of Ca(++), while C'1 disappears first in the absence of Ca(++). A large part of this C'1 disappearance is due to spontaneous inactivation. Similarities are pointed out between the influence of Ca(++) on the inactivation of the components of complement by antigen-antibody systems and by plasmin.