The lipopolysaccharides (LPS) from Klebsiella 03 and 05 and Escherichia coli 08 and 09 are unique in having linear homopolysaccharides consisting of mannose as the O-specific polysaccharide chains. All four kinds of LPS were found to exhibit very strong adjuvanticity in induction of delayed-type hypersensitivity to ovalbumin in mice compared with other kinds of LPS from Klebsiella, E. coli and Salmonella. Even if the natural forms of Klebsiella 03 LPS and 01 LPS were converted to various defined uniform salt forms, their adjuvanticity did not differ significantly from that of the respective natural forms. It was concluded therefore that the difference in strength of the adjuvanticity between them is not due to the difference in their salt forms, solubility and physical state. Correspondingly, with strong adjuvanticity of Klebsiella 03 LPS and 05 LPS, their activity in enlarging the regional lymph node was also strong. Various uniform salt forms of Klebsiella 03 LPS caused stronger regional lymph node enlargement than those of Klebsiella 01 LPS. However, the activity of E. coli 08 LPS and 09 LPS in enlarging the regional lymph node was significantly weaker than that of Klebsiella 03 LPS and 05 LPS, and there were other kinds of LPS which showed a capacity to enlarge the regional lymph node similar to that of E. coli 08 LPS and 09 LPS, despite their weak adjuvanticity. Therefore, correlation did not necessarily exist between the degree of adjuvanticity of LPS and its activity in enlarging the regional lymph node.