Summary and Conclusions: The use of a spectrophotometer in determining the percentages of hemolysis obtained in the titration of complement is described. The mathematical treatment of the curve of hemolysis is reviewed, and a method described for the preparation of graphs with the aid of which the 50 per cent hemolytic unit of complement may be determined directly from the percentages of hemolysis observed in a given titration without constructing the curve of hemolysis. The accuracy of such determinations of the 50 per cent unit was calculated for 13 titrations by reference to the standard unit determined by the method of least squares. The mean percentage of error was found to be +0.018, with a standard deviation of 0.29.