In a natural outbreak of respiratory disease during 1976, 31 of 43 calves showed moderately severe clinical signs which included pyrexia (maximum 42 degrees C) and tachypnoea. During the outbreak infection by respiratory syncytial virus (RSV) was demonstrated by isolation from nasopharyngeal swabs or by serology. Pasteurella haemolytica or P multocida were isolated from the blood of four and five calves respectively. In the month before disease 65 per cent of the calves showed significant antibody responses to P haemolytica. However a similar serological response to P haemolytica was demonstrated during 1975 in a comparable group of calves in which no disease and no infection with RSV was detected. No serological response to P multocida was demonstrated in either year. The temporal correlation of RSV infection with respiratory disease in the group of 43 calves was striking but the evidence neither reinforced nor discounted the possibility of interaction between RSV and P haemolytica infection in the pathogenesis of disease.