The hypothesis is discussed that dissociable complexes are formed between elements of a test complement (C') and the reagent used to titrate it for a particular component.

Calculations are presented which demonstrate that such complex formation may give considerable changes in slopes of percentage hemolysis-log dose regression lines obtained with mixtures of C' and reagent.

It is shown that marked change of slope may occur with relatively little change in the value of the 50 per cent intercept.

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