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Using potentially out-of-group blood components, like low titer A plasma and O whole blood, in the resuscitation of trauma patients is becoming increasingly popular. However, very little is known whether the donors’ anti-A and/or anti-B titers change over time and whether repeated titer measurements on the same donor are required to ensure that each donation produces a low titer product.The anti-A and/or anti-B titers were measured on 56 healthy adult volunteers (47 blood donors; nine blood center personnel) every 3 months for 12 consecutive months using an automated solid phase analyzer. The results were expressed as log2 titer steps (e.g., titer 32 = 5 titer steps).Minor variations in the average anti-A and/or anti-B titers were seen over time; the maximum individual SD in each group was 1.50 (IgG anti-A) or 1.00 (IgM anti-A, IgM, and IgG anti-B). When the SDs for the four titer measurements from all 56 volunteers were combined as appropriate, the highest overall combined SD was 0.47 titer steps for IgG anti-A. This value corresponds to a 95% confidence interval for intraindividual variation in this antibody’s titer over 12 months of 0.96 titer steps. Thus, based on one measurement, an IgG anti-A with a titer step of, for example, 6 would be expected to be in the range of titer step 5 to titer step 7 over the course of 1 year with 95% probability.The titers of anti-A and/or anti-B among healthy adults are stable over at least 1 year. This suggests that repeated titer measurements within a year on the same donor are not necessary if donations are made at 3 months or longer intervals.Diagnostic study, level V.