Risk of second antibody formation, depending on the antibody formed at first immunization in n = 643 patients for whom chronological analysis of immunization could be performed. Relative risk (RR) compared with that for all immunized patients, with 95% confidence interval (CI). When the first immunization included a WAA or NOA (RR and CI < 1), the risk was reduced compared with that in other patients; when the first immunization included an antibody to RH-K, AEP, or antibody to a HFA, the risk was higher (RR and CI > 1); the risk of second immunization after a first immunization including an AUS or antibody to a LFA was not significantly altered. The RR was highest when the first immunization included an antibody to an HFA. Those included anti-Fy3 (n = 8), anti-Fy5 (n = 4), anti-U (n = 4), anti-Co^a, anti-Js^b, and anti-Jo^a, (n = 1 each), and n = 8 not identified at first detection.