The thirteenth edition of the standards of the American Association of Blood Banks did not require the use of A1 red cells (RBCs) or an indirect antiglobulin test (IAT) to detect anti-A in neonatal serum, whereas the fourteenth edition mandates both. The present study was conducted to help document the need for these changes. Incomplete expression of the A antigen in neonatal patients can contribute to the accumulation of unabsorbed maternal anti-A that is capable of mediating the immune destruction of transfused adult RBCs. Sera from 50 group A neonatal infants of group O mothers were tested for anti-A by using RBC segments of A1 and A2 units of AS-1 RBCs less than 14 days old and also with A1 reagent RBCs. Of 22 sera in which anti-A was detected by RBCs from the A1 units with an IAT, anti-A was detected by RBCs from the A2 unit in only 1 and by the reagent RBCs in 17. In 19 (86%) of the 22 neonatal patients in whose sera anti-A was detected, the antibody was found only with the use of anti-human globulin in the IAT. It is concluded that testing to detect circulating anti-A in neonatal patients should include an IAT, and that it is preferable to use A1 RBCs for the initial evaluation.