High and moderate titer anti-A in O plasma inhibits platelet light transmission aggregation from a group A normal donor. Platelet rich plasma (PRP) samples with a platelet count of 250 × 103μL were mixed at a ratio of 9:1 with group A plasma, group O plasma of low (1:4), moderate (1:128), or high (1:1024) anti-A titers. In comparison to aggregation after exposure to A plasma (78%), inhibition was seen after exposure to moderate and high titer O plasmas (60% aggregation; inhibition of 23%; and 30% aggregation; inhibition of 62%, respectively). Aggregation with low anti-A titer O plasmas showed no differences from A plasma control (80%).