Application of the RHG method for quantification of complement (C′) is described. The measurements of potency obtained by the radial hemolysis in gel plates for different lots of C′ agree with the values obtained by conventional tube titration. With properly standardized plates, the RHG method can determine the CH50 units of different lots of C′ using an experimentally determined area of lysis (y) value obtained for CH50/ml. This also allows the estimation of residual CH50/ml after complement fixation by antigen-antibody. © 1973.