Isogenic non-encapsulated (K-) mutants (O1:K-) were obtained from several different Klebsiella pneumoniae O1:K1 serotype strains. By employing K. pneumoniae bacteriophages FC3-1, FC3-2, and bacteriophage phi 1, the bacterial surface receptors of which, are the O1-antigen of lipopolysaccharide and the K1 capsular polysaccharide (K1) respectively, the K1 polysaccharide was found to completely cover the O1 lipopolysaccharide molecules in each of the O1:K1 strains examined. Exposure of the O-antigen at the cell surface was only observed after growth in the presence of sub-minimum inhibitory concentrations of antibiotics. The implications of these findings for the design of vaccines for the prevention of Klebsiella infections is discussed.