The authors studied an example of red cell anti-JMH (John Milton Hagen) that exhibited several characteristics of a possible clinically significant antibody able to cause red cell destruction in vivo. Strong serological reactivity, a positive monocyte monolayer assay, and immunoglobulin subclass determination as IgG3 all indicate possible ability to destroy red cells. A 51Cr-labeled red cell survival study was not done as the patient did not require red cell transfusions but may be recommended for this patient if transfusions become necessary.