The specificity of the antibody to be titered is first determined and optimal phases of reactivity are selected. Two-fold serial dilutions of serum are tested against antigen-positive erythrocytes under optimal conditions. The result is the reciprocal of the highest dilution at which macroscopic agglutination (1+) is observed. Parallel titration of a previous specimen of the patient's serum (frozen) provides a baseline for comparison of antibody level. In the absence of a previous specimen from the patient, parallel titration of a control antiserum is used for standardization.