Abstract: The presence of aldehydic groups on proteins and lipoproteins is associated with various pathological conditions such as atherosclerosis, diabetes and alcoholic liver disease. Respiratory syncytial virus (RSV) is a major cause of severe respiratory disease in infants and the elderly. RSV vaccine research has been impeded because a formalin-inactivated vaccine used in the 1960s predisposed infants to enhanced disease following subsequent natural infection. The molecular basis for the vaccine-induced hypersensitivity has not, however, been elucidated. We show here that addition of reactive carbonyl groups to ovalbumin (OVA) by treatment with glycolaldehyde or formaldehyde increases the protein's immunogenicity in mice, and biases the immune response towards a Th2-type response. The increased immunogenicity and the Th2-type response can both be abrogated by reductive elimination of the reactive carbonyl groups.