Basophil histamine release in a patient with an IgE-mediated seminal plasma allergy had a requirement for serum. Washed leukocytes, in the absence of serum, released little histamine on challenge with seminal plasma antigen. The addition of serum markedly enhanced the release. However, serum had only a mild effect on ragweed antigen induced histamine release from the same cells of this individual. Serum from normal donors was equally effective as autologous serum. Heating the serum and treating it with mercaptoethanol did not destroy this activity. The serum effect was unique in that another patient with seminal plasma allergy did not demonstrate this phenomenon. It is postulated that the effect of the serum is to stabilize the antibody-antigen step at the basophil surface.