The potential role of respiratory infections in altering the development of atopy and asthma is complex. Infections have been suggested to be effective in preventing the induction of T-helper 2-polarized allergen-specific immunity in early life, but also to exacerbate asthma in older, sensitized individuals. The mechanism(s) underlying these effects are poorly defined. The aim of this work was to determine the influence of lipopolysaccharide (LPS) exposure on the development of sensitization to allergen and the response to allergen challenge in vivo. Piebald-Virol-Glaxo rats were exposed to a single aerosol of LPS 1 d before or 1, 2, 4, 6, 8, or 10 d after sensitization with ovalbumin (OVA). On Day 11 animals were exposed to 1% OVA and responses to allergen were measured 24 h later, monitoring inflammatory cell influx and microvascular leakage into bronchoalveolar lavage (BAL) fluid as well as pulmonary responses to methacholine using the forced oscillation technique. Histologic analysis was included to complement the BAL results. Single aerosol exposure to LPS 1 d before and up to 4 d after intraperitoneal injection of OVA protected against the development of OVA-specific immunoglobulin (Ig) E. LPS exposure 6, 8, or 10 d after sensitization further exacerbated the OVA-induced cellular influx, resulting in neutrophilia and increased Evans Blue dye leakage with no effect on serum IgE levels. In addition, LPS abolished the OVA-induced hyperresponsiveness in sensitized animals when given 18 h after OVA challenge. This study demonstrates that exposure to LPS can modify the development of allergic inflammation in vivo by two independent mechanisms. Exposure early in the sensitization process, up to Day 6 after exposure to allergen, prevented allergen sensitization. Exposure to LPS after allergen challenge in sensitized animals abolished the hyperresponsiveness and modified the inflammatory cell influx characteristic of late-phase response to allergen.