Summary: There is no other means of measuring the specific agglutinin content of a serum except by using the bacterial type which is known to have stimulated the production of the agglutinins in a serum even though this means may not always be adequate. There is no basic validity therefore, in a procedure which assumes the reliability of determining the infecting bacterium or bacteria by testing the absorptive capacity of several cultures on the agglutinins present in the patient's serum for each of such cultures. Such a procedure does not take into account all the possible alternatives. The observation of the character of the clumping and the use of the specific component for test purposes may give strongly suggestive results. The probable alternatives in such methods are discussed.