Two commercially available enzyme immunoassays and one assembled in house were used to measure immunoglobulin M (IgM) antibody to cytomegalovirus (CMV) in a total of 220 serum specimens from 104 renal transplant recipients. All assays included a step in which interfering IgG antibody was removed or complexed. Concordance of results between pairs of assays ranged from 84 to 96%. All sera from patients with recent seroconversion (primary CMV infection) had measurable anti-CMV IgM. Among those already seropositive to CMV when transplanted, 26 to 55% had IgM antibody posttransplant, depending on the assay. This was observed regardless of the CMV serologic status of the kidney donor, indicating that reactivation of endogenous CMV, as well as reinfection, can induce this antibody in transplant recipients. Four cadaver donors known to transmit CMV to eight recipients did not have measurable IgM antibody to CMV.