BACKGROUND: Jra is a high‐frequency antigen seen in all populations, but the clinical significance of Jra antibodies is incompletely understood. Two cases are reported in which patients with anti‐Jra received incompatible transfusions.CASE REPORTS: A 69‐year‐old Japanese man had anti‐K and anti‐Jra. Despite multiple transfusions of Jr(a+), K– RBCs, his clinical course remained stable without evidence of hemolysis. A 45‐year‐old Japanese woman with anti‐Jra was transfused with two units of Jr(a+) RBCs without clinical evidence of hemolysis. However, the same patient received an additional unit of Jr(a+) RBCs 1 week after the initial transfusions and, within 6 hours of transfusion, developed signs and symptoms of an acute hemolytic transfusion reaction.STUDY DESIGN AND METHODS: Routine serologic methods were used to study the patients’ RBCs and plasma. A monocyte monolayer assay (MMA) was used to determine the potential clinical significance of the anti‐Jra, where reactivity (R) greater than 5 percent indicates potential clinical significance.RESULTS: The anti‐Jra in the first case had a pretransfusion titer of 32 with a MMA result of 3.3 percent R. No clinical or laboratory evidence of hemolysis was seen after transfusion of 4 units of Jr(a+), K– RBCs. The anti‐Jra in the second case had a pretransfusion titer of 32 with a MMA result of 24.5 percent R. This patient developed an acute hemolytic reaction after transfusion of Jr(a+) RBCs.CONCLUSION: Anti‐Jra can be clinically significant as demonstrated by acute hemolysis in the second case. The MMA accurately predicted the clinical outcome of each case and appears to be a useful tool in predicting the biologic behavior of anti‐Jra.