Cytomegalovirus (CMV) infections in renal transplant recipients must be diagnosed rapidly, since they can be life-threatening unless chemotherapy is started early. Detection of granulocyte-associated immunoglobulins was compared with conventional virological methods for diagnosis of CMV infection in 71 renal transplant recipients. The granulocyte-associated immunoglobulin test (GAIT) was positive in 31 of 34 patients with proven CMV infections on the day of admission. By conventional virological criteria the diagnosis of active CMV infection could be made only 3-24 days later. The GAIT remained negative in 14 healthy transplant recipients, but it was positive in 9 of 23 patients with non-CMV-related post-transplantation complications. The GAIT, which is not a virological method, could be useful for rapid diagnosis of CMV infection; its sensitivity was 0.91 and specificity 0.82 (for patients without detectable immunoglobulins on erythrocytes or platelets) and the processing time is only 3 h.