Numerous investigators have reported an increased incidence of pneumonia caused by Gram-negative bacilli and other secondary pathogens in transplant recipients infected by cytomegalovirus (CMV). To determine if CMV infections are related to colonization of the upper respiratory tract by Gram-negative bacilli, we examined prospectively 22 renal transplant recipients with sequential bacteriological, virological and biochemical examinations performed just prior to and at various times after transplantation. Only 11% of subjects had Gram-negative bacilli isolated from gargle specimens prior to transplantation, as compared to 54% after transplantation. More importantly, after transplantation, subjects with active CMV infections were more likely to have prolonged oropharyngeal carriage of Gram-negative bacilli than subjects without CMV infections (36% v. 25%). During active CMV infections, the rate at which Gram-negative bacilli were isolated from gargle specimens rose from 28 to 47%. During culture-positive CMV infections, the isolation rate reached 57% and was significantly different from that of CMV-negative samples (P less than 0.01). The increased rate of Gram-negative bacillary isolation from gargle specimens during CMV infections was not a function of type of immunosuppressive agents used, rejection episodes, antibiotic administration, concomitant hepatitis B, Epstein-Barr (EBV) virus, or herpes simplex virus infections, or alterations in salivary fibronectin concentrations.