A method is described for making serial estimations of serum complement. Titrations were made in a standardized haemolytic system and the 50 per cent haemolysis end point obtained by relating the probit percentage haemolysis to the reciprocal of the corresponding serum dilution. Possible variable factors such as changes in pH and red cell concentration were studied and it was found that the most important source of error arose from the use of different batches of sheep red cells. This was overcome by running a lyophilized standard reference serum in parallel with all titrations and expressing the unknown sera in terms of this standard. Observations were then made on a group of selected subjects to form the basis for a study of changes in patients suffering from renal disease.