We believe that sufficient evidence exists to justify 3 classes of Rh antibodies based on their reactivity. These are the classical agglutinin, the blocking antibody, and the proposed "cryptagglutinoid." Sufficient distinction exists to retain the blocking antibody as determined by Wiener’s original blocking test. This antibody is further characterized by failure to act in Chown’s capillary technic. The identity of this antibody with its important role in reawakening investigation in this field of immunology should not be lost through inclusion in the broader group of those antibodies determined by the albumin test and similar methods. The proposed third order antibody or "cryptagglutinoid" which is usually, but not always, detected by these more inclusive tests, and the capillary technic, appears to be of great clinical importance because of the frequency with which it appears in significant titres. Detection of isoimmunization and closer correlation of antibody titre with clinical severity of disease should be possible through study of this "cryptagglutinoid" antibody.