Cytomegalovirus-specific in vitro antibody production (CMV-IVAP) by peripheral blood lymphocytes (PBL) was investigated in 12 renal transplant recipients. CMV-IVAP, CMV-serology, and viral cultures were carried out weekly during at least 8 weeks after transplantation. Nine episodes of CMV infection occurred in eight patients. CMV-IVAP was positive in eight episodes; IgG and/or IgA and/or IgM antibodies reacting with several CMV polypeptides were secreted by PBL. In two patients with primary CMV infection, only IgA antibodies were detected. Cytomegalovirus cultures were positive in eight episodes and serological evidence of CMV infection was obtained in five episodes. In one case, CMV-IVAP was observed before CMV isolation and in another case, before serological evidence of CMV infection. Our study indicates that CMV-IVAP could represent an additional tool for the diagnosis of CMV infection. Our study indicates that CMV-IVAP could represent an additional tool for the diagnosis of CMV infection in renal transplant recipients.