To investigate whether immunization with Escherichia coli J5 boiled cells induces antibodies directed at deep core structures, antibodies against J5 lipopolysaccharide (LPS), Re LPS, and lipid A were measured in the serum of 70 volunteers before and 2 weeks after immunization. To improve the sensitivity and the specificity of ELISA, complexes of core LPS with high-density lipoproteins were used instead of free core LPS as antigens. A median three-fold increase in antibodies directed against J5 LPS was observed, but no significant increase in the antibodies against Re LPS or lipid A was found. Since J5 antiserum did not react with several smooth LPS or with Re LPS and lipid A, cross-reactivity could not be demonstrated. Thus, immunization of volunteers with E. coli J5 produced a modest specific antibody response against J5 LPS. The mechanism of protection previously observed with J5 antiserum remains unclear.