In 1912 Dean (1) showed that the relative proportion of antigen and antibody is of the greatest importance in both precipitation and complement fixation reactions. He showed, conclusively, that to determine quantitatively the antibody content of a serum or the antigen content of a serum or extract, it is necessary to titrate decreasing quantities of antiserum with decreasing amounts of antigen. The significance of this work does not seem to have been fully recognized although its practical bearing on any experiments dealing with precipitation or complement fixation tests is of the utmost importance. In fact, if the quantitative relation of antigen and antibody is not understood, especially in complement fixation reactions, antibodies in a strong serum or antigen in a strong extract may be entirely missed, or the opposite may occur, that is, antibodies in a weak serum, or antigen in a weak extract, may be overlooked.