A-anti-A, like G-anti-A and unlike M-anti-A was unaffected by heating at 56°C for 3 hr. On the other hand in the following three characteristics the behavior of A fell between that of G- or M-anti-A: sensitivity to inactivation by 2-mercaptoethanol, ease of neutralization by A substance and degree of enhancement of agglutination in a medium of serum rather than saline. The agglutination produced by A-anti-A was regularly enhanced by addition of anti-A-globulin serum.

In searching for A-blood group antibodies of other specificities the following sera were tested: anti-D (32 examples); anti-c (2 examples); anti-Le^a or -Le^b (3 examples); anti-K (3 examples); anti-Fy^a (3 examples), and anti-Jk^a (3 examples). Only 3 sera, all containing anti-D, sensitized red cells to agglutination by anti-A. There were no discrepancies between results obtained with four different anti-A-globulin sera. Approximately half the sera were fractionated on DEAE-cellulose, and the fractions rich in A tested for their ability to sensitize red cells to agglutination by anti-A; no additional examples of A-antibodies were detected.

One of the three examples of A-anti-D appeared in the serum of a woman during the course of deliberate reimmunization. A-anti-D appeared only after three intravenous injections of red cells although the G-anti-D titer rose considerably after a single injection. 3 yr after a fourth injection of Rh-positive cells A-anti-D, as well as G-anti-D, was still present in the serum.