A modification of the spectrophotometric method of complement assay described by Kent (17) is outlined for determining objectively the optimal level of haemolysin to be used in the complement-fixation test. The haemolytic curves obtained by assaying haemolysin at a number of levels are described by a family of linear regression lines (logarithm of the dose of complement-probit percentage of haemolysis) that have a common origin but differ in slope. The optimal level of haemolysin is chosen as that dose which produces the maximum sensitivity of the erythrocytes to the action of complement as manifested by the steepest line.

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