To determine whether latent sensitivity to respiratory syncytial virus antigen(s) occurs after infection, 27 infants with acute bronchiolitis were studied and compared with 15 hospital controls. Blood was collected for whole blood challenge, and histamine release was measured by a high performance liquid chromatography technique with fluorometric detection. There was a significantly greater histamine release to respiratory syncytial virus antigen(s) in those with bronchiolitis than in controls, expressed either in amount (median 154 nmol/l compared with 104 nmol/l) or percentage release (median 20% compared with 3%). There was a significant difference between index and control groups in terms of individual histamine responses. These findings strongly suggest that infants develop latent sensitivity to respiratory syncytial virus antigen(s) during the course of acute bronchiolitis. Serial lung function tests were performed in 15 infants. All infants had abnormalities of lung function at some stage, but the small numbers of subjects precluded comparison between 'sensitised' and 'non-sensitised' infants. Further study is indicated to define the relation of latent sensitisation and subsequent bronchial hyper-responsiveness after respiratory syncytial virus infection in infants.