The possible involvement of cytokines in the acute viral pneumonia induced by respiratory syncytial virus (RSV) infection was studied in calves. The patterns of cytokine mRNA expression in mononuclear cells (MNC) isolated from the lung and peripheral blood of six gnotobiotic calves infected seven days previously with bovine RSV were analysed by reverse-transcription polymerase chain reaction using primers specific for bovine cytokines. The pattern of cytokines detected indicated a mixed type of response to RSV infection as mRNAs for IFN-gamma, IL-2, IL-4 and IL-10 were detected in pulmonary and peripheral blood MNC from calves with extensive pneumonic consolidation. In contrast, only mRNA for IFN-gamma was detected in MNC from the lungs and peripheral blood of uninfected animals. These data provide preliminary information on the potential range of cytokines produced in calves following infection with bovine RS virus.