Changes in antibody titer against the Epstein‐Barr virus (EBV), the apparent cause of infectious mononucleosis, have been surveyed in 30 patients, randomly selected from kidney transplant recipients in Aarhus during the last five years. Sera taken at transplantation and at approximately 1, 2, 3 and 5 to 12 months post‐transplant were examined for antibodies against EBV using an indirect immunofluorescence technique with EB‐3 cells. Anti‐EBV antibody, and thereby evidence of past infection, was found in all but one set of sera. In three patients a significant, fourfold, rise in antibody titer was observed. One, a man of 31, had a transient illness with fever, sore throat and enlarged lymph nodes in association with antibody changes; another, a girl of 18, had two snort febrile periods of unexplained etiology; the third. a girl of 11, had no recorded symptoms.