Abstract Background: Endotoxin/LPS is ubiquitous in our environment. The question whether lipopolysaccharide (LPS) is beneficial or disease-promoting in relation to asthma and allergy has been raised in several recent studies. Some have reported a positive correlation between the level of LPS in house dust and the symptoms of asthmatic children. Others have found that exposure to LPS appears to protect against the development of atopic disease in children. Objectives: We performed a study in mice to examine the antibody response after subcutaneous immunization with LPS and the cat allergen Fel d 1. We asked whether LPS would increase the response and direct the antibody production towards an allergic (IgE), or non-allergic (IgG2a) antibody profile. In rodents both IgE and IgG1 are antibodies produced under Th2-dependence and IgG2a antibodies under Th1-dependence. Also, when LPS and Fel d 1 are introduced to the immune system, we asked whether the timing of the two agents relative to each other is crucial. Methods: The mice were injected subcutaneously with LPS and/or Fel d 1 four times in various orders. IgE, IgG1 and IgG2a antibodies specific to Fel d 1 were measured in serum using ELISA. Results: A strong antibody response, both for IgE, IgG1 and IgG2a, was observed only when Fel d 1 and LPS were injected simultaneously, and in particular after repeated injections. Conclusion: A strong specific antibody response was observed, both for IgE, IgG1 and IgG2a, only when LPS was introduced to the immune system together with the cat allergen Fel d 1. No such adjuvant effect was observed when LPS was introduced alone prior to or subsequent to the allergen. The resulting antibody response was not polarized in terms of Th1- or Th2-dependence.